I think it would help to redesign DiaFontSelector, so that the "other" menu option opens the Gtk dialog for fonts, just like the color selector opens the Gtk dialog for colors. This way, we can do away with the hardcoded list of fonts in widgets.c, and also leave things up to Gtk, rather than complicating our work.

that hardcoded list has nothing to do with the widget displaing fonts. And it has nothing to do wit the length of the list. The hardcoded list is just for backward compatibility: mapping legacy font names to their Pango representation.
Back to topic: there are different Pango font backends for win32. Gimp is using Pangoft2's font list (including e.g. bitmap fonts), but just for it's text tool.

Dia can be compiled for both variants - pangoin32 being the default on win32. It would be a bug in Dia if the standard list of fonts in Gtk's font dialog (win32)would not match the  (win32) list presented by Dia. Everything else is more likely a known limitation of pangowin32 (e.g. no bitmap fonts) or a misconfiguration/regression in Pango. Btw. my font list in Dia is more than three screens height. I don't consider that too short.

For me, it only contains

Arial
Arial Black
Comic Sans MS
courier
Courier New
Geogia
Impact
Lucide Console
Lucida Sans Unicode
Microsoft Sans Serif
monospace
Palatino Linotype
sans
serif
Tahoma
Times New Roman
Trebuchet MS
Verdana

While dir c:\winnt\fonts\*.ttf gives

19.06.2003  21:05              367.112 arial.ttf
19.06.2003  21:05              352.224 arialbd.ttf
04.10.2000  14:00              221.756 arialbi.ttf
04.10.2000  14:00              200.684 ariali.ttf
04.10.2000  14:00              117.028 ariblk.ttf
04.10.2000  14:00              126.364 comic.ttf
04.10.2000  14:00              111.476 comicbd.ttf
04.10.2000  14:00              297.660 cour.ttf
04.10.2000  14:00              306.688 courbd.ttf
04.10.2000  14:00              228.796 courbi.ttf
04.10.2000  14:00              238.088 couri.ttf
04.10.2000  14:00              142.964 georgia.ttf
04.10.2000  14:00              139.584 georgiab.ttf
04.10.2000  14:00              156.668 georgiai.ttf
04.10.2000  14:00              158.796 georgiaz.ttf
04.10.2000  14:00              136.076 impact.ttf
04.10.2000  14:00              115.068 lucon.ttf
04.10.2000  14:00              323.980 l_10646.ttf
05.06.2007  00:00               59.776 Mathematica1.ttf
05.06.2007  00:00               56.740 Mathematica1b.ttf
05.06.2007  00:00               59.964 Mathematica1m.ttf
05.06.2007  00:00               54.312 Mathematica1mb.ttf
05.06.2007  00:00               45.524 Mathematica2.ttf
05.06.2007  00:00               44.536 Mathematica2b.ttf
05.06.2007  00:00               47.104 Mathematica2m.ttf
05.06.2007  00:00               44.456 Mathematica2mb.ttf
05.06.2007  00:00               51.444 Mathematica3.ttf
05.06.2007  00:00               54.772 Mathematica3b.ttf
05.06.2007  00:00               49.124 Mathematica3m.ttf
05.06.2007  00:00               48.068 Mathematica3mb.ttf
05.06.2007  00:00               57.600 Mathematica4.ttf
05.06.2007  00:00               53.692 Mathematica4b.ttf
05.06.2007  00:00               56.892 Mathematica4m.ttf
05.06.2007  00:00               54.216 Mathematica4mb.ttf
05.06.2007  00:00               67.364 Mathematica5.ttf
05.06.2007  00:00               65.924 Mathematica5b.ttf
05.06.2007  00:00               59.376 Mathematica5m.ttf
05.06.2007  00:00               59.352 Mathematica5mb.ttf
05.06.2007  00:00               26.868 Mathematica6.ttf
05.06.2007  00:00               27.316 Mathematica6b.ttf
05.06.2007  00:00               21.420 Mathematica6m.ttf
05.06.2007  00:00               21.384 Mathematica6mb.ttf
05.06.2007  00:00               22.440 Mathematica7.ttf
05.06.2007  00:00               22.176 Mathematica7b.ttf
05.06.2007  00:00               23.628 Mathematica7m.ttf
05.06.2007  00:00               23.132 Mathematica7mb.ttf
19.06.2003  21:05              259.384 micross.ttf
04.10.2000  14:00              506.836 pala.ttf
04.10.2000  14:00              451.232 palab.ttf
04.10.2000  14:00              362.168 palabi.ttf
04.10.2000  14:00              448.112 palai.ttf
04.10.2000  14:00               69.464 symbol.ttf
19.06.2003  21:05              265.528 tahoma.ttf
19.06.2003  21:05              260.472 tahomabd.ttf
19.06.2003  21:05              409.280 times.ttf
19.06.2003  21:05              398.372 timesbd.ttf
04.10.2000  14:00              233.892 timesbi.ttf
04.10.2000  14:00              241.972 timesi.ttf
04.10.2000  14:00               65.100 trebuc.ttf
04.10.2000  14:00               65.300 trebucbd.ttf
04.10.2000  14:00               65.296 trebucbi.ttf
04.10.2000  14:00               71.320 trebucit.ttf
04.10.2000  14:00              139.640 verdana.ttf
04.10.2000  14:00              136.032 verdanab.ttf
04.10.2000  14:00              154.264 verdanai.ttf
04.10.2000  14:00              153.324 verdanaz.ttf
04.10.2000  14:00              118.752 webdings.ttf
04.10.2000  14:00               81.000 wingding.ttf

I assume that a *.ttf font cannot be a bitmap font.
Any ideas what be going wrong?

Yes, looks like the only additional fonts you have are symbol fonts. As noted above they are probably not available in *any* program using pangowin32. See:

2004-09-15  Tor Lillqvist  <tml@iki.fi>

	* pango/pangowin32-fontmap.c (pango_win32_insert_font): Ignore
	fonts in SYMBOL_CHARSET. They don't have any Unicode mapping
	table. (#149643)

I think there is some implicit unicode mapping given by the SYMBOL_CHARSET constant, but it would need to be implemented in Pango. Please check as suggested in my previous comment.

Thanks for the information. The OpenSymbol font included with OpenOffice
seems to solve the problem because it includes a Unicode mapping.