I have not done this so if you go this route test it first, but I would think a sanding sealer woud do it. Mylands has a sanding sealer that is sold at various wood places. I have to order it off the Internet. I'm sure there are other brands. I have used this on lots of other woods with success. It really pops the grain.
I have seen someone use joint compound tinted to match the wood to fill pores on an acoustic. They rubbed it into the pores, quickly rubbed the execess off, and then sanded it smooth as a babies bottom.
You can use fine wood saw dust mixed with lacquer to the consistency of a paste, then it rubbed in the pores, let dry and sanded with 400 grid sandpaper. That should give you good surface for the finish, and it will meatch the wood since you use the same wood for saw dust.
I collect the saw dust from my sander.
I'll reply even though this is 10 years old. .....Maybe it can help this poor guy still looking for a solution. LOL.
I have tried every trick to grain fill Mahogany and have now settled on super glue AKA CA glue. Give it a skim coat with an old credit card type thing, then sand with 320 when dry. Finish with top coat of your choice. Glass like smooth every time.