I don't think bleach is possible. One of the tests for identifying an old platinum print over a silver one involved attempting to bleach out a spot in the corner. Silver would bleach. Platinum would not. Platinum is very resilient to any chemical reaction you'd want to expose a print to (It's what makes them archival).
There were Kodak (and other) strips for whiting out spots (I think it worked like carbon paper). I've not tried it to know if it'd work with platinum but I'll try to dig out the retouching stuff I got in a auction bin and see if it works on a scrap platinum.
Prismacolor color pencils do not work IMHO. They are far too waxy, and shiny - they stick out like a sore thumb. In general a platinum image is such a delicate part of the paper surface that retouching is a delicate operation. I think hard pastel pencils might work but the white ones are not very opaque.