What do you think of the notion of allowing an admin to permit any size image to be uploaded in mem_moderation_image? It's possible to not force a default 640x480 in modimg_get_image_dimensions() such that if you clear out the 'Image Size' pref then the entire resize/crop action in mem_image_save() can be skipped with a conditional test.
Advantage: smd_thumbnail (or equivalent) can be employed to automatically create multiple image sizes and can also be used after upload to create more images from the full-size original that has been moderated.
Possible disadvantage: any dimension image (may be gigantic) can be uploaded, up to the size as defined in Txp's Max Image File Size pref. Caveat utilitor?
The same logic could be applied to the thumbnail dimensions if desired.