With a simple change to Mongoid::Collection I've been able to dynamically change the database at runtime. The change is in the master method of collection so it now looks like this (all other methods removed of course):

module Mongoid
class Collection
def master(options = {})
options.delete(:cache)
db = Mongoid.databases[@klass.database] || Mongoid.master
@master = (@master.nil? || db.name!=@master.db.name) ? Collections::Master.new(db, @name) : @master
end
end
end

instead of this:

module Mongoid
class Collection
def master(options = {})
options.delete(:cache)
db = Mongoid.databases[@klass.database] || Mongoid.master
@master ||= Collections::Master.new(db, @name)
end
end
end

Unfortunately the specs in collection_spec fail after this change and I haven't figured out a good way to make them pass without uglifying the specs. I don't really think this poses any problem but are rather things to do with mocks and expectations. Putting out there to get some feedback since I'm not that familiar with the mongoid codebase.