Warren released some information about the new PyMol, 0.99beta11.
In 0.99beta11 and beyond, the newly implemented command to carry this out is "map_trim". (Windows build at http://delsci.com/beta or compile from source).
map_trim map-name, selection-name, buffer
Combined with the convenient new object-name wildcards (!!!), you could for example trim all your maps to 3 Angstroms around ligands with one commmand as follows
map_trim *, organic, 3
With map size now reduced, the map_double command can come in handy to increase mesh density on your figure.
Also in this new beta: Dunbrack rotamers, nucleic acid cartoons, new pop-up menus, etc.