This is closely related to Bug 629474 and Bug 629474 , and GdkPixbuf and its lack of a non-incremental loader (or requirement of an incremental one) is to blame. 

But it *is* possible for libgnome-desktop to access GdkPixbuf in a different way, allowing thumbnails for icns files to be generated.

For local files it is pretty trivial: instead of (or as a fallback to) using Loaders, file info, GLib's Icon preview, GStreams, etc, simply open the file, resize it and save.

Here is a tiny (and working) example in Python:

#!/usr/bin/env python
import sys
from gi.repository import GdkPixbuf
inputname, outputname, size = sys.argv[1:]
pixbuf = GdkPixbuf.Pixbuf.new_from_file(inputname)
scaled = GdkPixbuf.Pixbuf.scale_simple(pixbuf, int(size), int(size),
                                       GdkPixbuf.InterpType.BILINEAR)
scaled.savev(outputname, 'png', [], [])

A more complete solution is available at:
https://github.com/MestreLion/icns-thumbnailer

So, if a 8-line python script can generate a thumbnail with current GdkPixbuf, with all its limitations, there's no reason for libgnome-desltop not to do so too.

Please make it happen
Cheers,
ML