How about letting map designers split the maps themselves.
Obviously they'd have to make sure that the maps still fit together
nicely without any gaps but that shouldn't be too diffcult.
Well, Alex has to answer this (as he'll have to code it), but in general
irregular maps would be much harder to handle. For example, a map no
longer has exactly 1 adjacent map on each side.
As for the naming, I guess you wouldn't have to name them at all. The
designer would work at a single world map and would hardly notice that
stuff is split up by the editor.