If I may ask, what are you trying to accomplish?
MongoDB is not really a lightweight database server in the sense the Omega is lightweight. For example if i recall correctly MongoDB starts off with 100 MByte initial size for an empty database. It is really more intended to run on servers than on embedded systems.
Maybe Redis woudl be a better choice (not sure)? Or what about somethin embedded like SQLite of BerkleyDB? Or maybe using Micropython in conjunction with their BTree module?