Nope, sorry. And don’t have many ideas also how you could make it client proof with the current MaxiGallery.
You could use phx:phpThumb modifier if that offers better help at creating the thumbnails automatically..
Or, somewhat stupid idea, but you could create two galleries or three actually. One where client uploads the pictures and another one where client uploads the thumbnails. Then third one what shows the gallery and it gets the thumbnail from a gallery in doc1 and picture from a gallery in doc2.

But that would only work if the thumbnails and pictures are in same order in both galleries... and you’d have to set it pretty rigid with the document ids and all, so it would not be easy for a client to create a new gallery by himself.. so this kind of setup would possibly lead to alot of problems.. Then again, if you have some maintenance contract with your client..