Somebody with a IIgs just needs to try it out! 
If it doesn't already work, I think it could be done. Aside from the image file format, the other place where things might need adjustment is in the headers, footers, and checksums for each sector. I know the the IIgs uses the same method to read and write bits from the floppy, but it may still use a different header signature than the Mac, or have a different sector size, or a different checksum algorithm, or something like that. That stuff isn't part of the disk image file - it's generated on the fly by the Floppy Emu firmware.
A cable from the rear to the front is a handy accessory. You can buy one here: http://www.iec-usa.com/cgi-bin/iec/L1561
If it doesn't already work, I think it could be done. Aside from the image file format, the other place where things might need adjustment is in the headers, footers, and checksums for each sector. I know the the IIgs uses the same method to read and write bits from the floppy, but it may still use a different header signature than the Mac, or have a different sector size, or a different checksum algorithm, or something like that. That stuff isn't part of the disk image file - it's generated on the fly by the Floppy Emu firmware.
A cable from the rear to the front is a handy accessory. You can buy one here: http://www.iec-usa.com/cgi-bin/iec/L1561