Your plate on a 3/4" shank idea is probably close to the best; the only change that might make it more rigid is to use a face mill holder and make a plate to fit this. There have been a few threads about putting a chuck on a mill spindle and I don't think anyone found a ready made solution.
Here is a link that takes you to one thread, these are dealing with larger size CNC mills:
http://www.cnczone.com/forums/showthread.php?t=51075
And here is one I did for fun which shows what is possible:
http://www.cnczone.com/forums/showth...398#post406398
An open mind is a virtue...so long as all the common sense has not leaked out.