Hi,
we created a tool in the source tree to split bson files into smaller pieces. We use it to dump data into hadoop. It also does LZO compression.

Would you be interested in pulling it into master?
https://github.com/foursquare/mongo/blob/r2.0.4-fs-yan/tools/bsonsplit.cpp

It looks like some other people had the same need:
https://groups.google.com/forum/?fromgroups#!topic/mongodb-user/N3rA8eyu9ms