An old thread but thought I would give a followup for anyone thinking of using a mirror to extend throw.
I took the plunge and ordered a 24"X12" first surface mirror from FirstSurfaceMirror.com. There may be one or two other sources on the internet but this site was referenced from other members...
I have set up my theater projector using a mirror to increase throw. A bit tricky because you have to mount the projector outside of the reflected image. You can use a standard 2nd surface bathroom type mirror to set things up before plunking down the cash to buy an optical grade 1st surface...