In go-format-sel.c we have:

/* TODO We should add the list of other custom formats created.
 *      It should be easy.  All that is needed is a way to differentiate
 *      the std formats and the custom formats in the GOFormat hash.
*/

ie. Currently the custom format page of the selector do not include any formats created by the user for this spreadsheet, but this would be very desirable.

We need to
1) add a field to GOFormat flagging it as build-in
2) set that flag when we create the standard formats
3) when we create the custom page, scan through the hash for non-build-in formats and add them.