it is possible but enormously difficult. it would be better and perhaps even easier to just build an all new mobo.
powerpc cpu > complex glue logic > pc mobo
a new bios wouldnt be necissary because you would have to have a flash rom on the board to start it up and it could easily be made to ignore what is on the mobo
the glue logic asics or whatnot would do all of the interfacing between different timing and singnals between cpu and mobo. this is very similar to what 030 accelerators that plug into the 68000 socket do but on a much bigger scale.
basicly what would would have is a very complicated card that is using a cpu socket as a weird external bus.