Happened just because...!
Having another look at MongoDB again with the alpha. Its quite a tricky library to import becaue it has Linq-style queries for finding for sifting, but they made their own probably for efficiency. Making a delegate with the right signature isn't enough to build a query so I'm suspecting I may have to either muck around with the forwarding/ the interfaces or a combination of both and other things.
However, for now wanted to know how to make custom regions.
An example signature of the actual where operation as as found in the project manager is:
Where<TSource>(IMongoQueryable<TSource> Source, Expression<Func<TSource, Boolean>> predicate): IMongoQueryable<Source>
I think I've seen an example somewhere but I can't recall where. Maybe at Praxis?