out of core property map and constant property map

Has anyone created property maps that operate out of core? either against a database or a flat file? Would you like to share? Also has anyone created a property map that, once create returns a constant value for all queries (writable is nice, it doesn't do anything with the write of course)? This might be useful for compactly representing constant edge capacities. Finally, an out of core BasicMatrixConcept implementation for use with all-pairs shortest path on largish (n=1.6M, e=10*n) graphs would be handy. Thanks, Joel