
Well, first of all I get very bent when it comes to magicians stepping in on Mentalist territory. I think it's a cheap PR shot most of the time. Secondly, I don't think the performer will benefit in doing such things because of how Mentalism invokes belief where Magic suspends it -- they are psychological opposites when it comes to how both work and those that see it as being "a trick" loose on multiple levels.
That said, you have a couple of simple options based on what you've described the best being that you do a bulk switch on the envelopes and secretly open the real ones, take some quick notes that you can conceal in some way (I usually use the top edge of a Legal Pad, where the paper tears off -- a Ted Lesley idea). The Dummy Slips should be destroyed during your performance however.
I'm hesitant to suggest the use of clearing liquids such as Ether, Alcohol and Dry Cleaning Solution but they are practical to your scenario, I'll leave it to you to figure out a method of application but would suggest you look up an old effect from the GEM Magic Co. here in the states called GEM SEE or check out Borodin's Tray in "Final Curtain".
NOTE: if you use clearing fluids during real time, you will want scented candles or some other olfactory "Misdirection" because of the smell said chemicals give off. Too, if you use ether, be exceptionally careful. . . it will knock you out.