Upload & Sell: On
So ... where do I start?
(Disclaimer: I take no responsibility... at your own risk... blah, blah...)
1) Try to accurately measure your 5D mirror.
2) Go to pawn shops or used camera stores and measure the mirrors of cheap, old analog cameras. The more manual they are, the more likely to have 100% silvered mirrors. Choose one that is big enough and as close to the same thickness as possible to the 5D, but thinner is better than too thick.
3) Carefully remove the mirror from your 5D. It is held in place with adhesive, AFAIK. Gentle, constant pressure/prying in very warm conditions is your best bet. This is the part that you want to take your time with!
4) Compare the mirrors and then carefully reduce the size of the replacement mirror to the same size as the original (if necessary).
5) Install the replacement mirror using a thin, uniform layer of appropriate adhesive. Preferably one that does not "cure" into a hard form, but rather maintains some elasticity to withstand the actuations and allows removal/replacement later.
6) Re-calibrate your focusing screen to compensate for any difference in mirror thickness or roughhousing of the mirror mechanism during the process.
This is just a brainstorm, but it is how I would likely go about it.
I have also played with the idea of simply applying an extremely thin layer of 100% reflective silvered material to the existing mirror, but I never found the perfect product to do so.