I did some googling:
If you want to view the maps already in DG, you can download the granny viewer for free, but AFAIK you can't export to another format for editing, nor can you edit anything from eithin that program, nor can you import from a standard .fbx or .x or whatever the flavor of the month for mesh formats is.
So the format we are dealing with for the meshes is .gr2, which is unfortunately not open.
There was a post asking how to do this on gamedev.net, which is generally a good source, but the response was rather negative.
Fortunately, many games are using gr2, such as AoE3 and neverwinter nights 2
The latter is definately moddable, so looking into the tools people have made to muck around with NWN2 we can find...
"Based on what i've seen from Tazpn's source code for 3ds max... gr2 import/export should be... not easy... but possible. He's basically laid out the groundwork for re-implementing expotron by using the granny.dll as a black box for file i/o."
I don't have time to play with this further, but once my exams are over in a couple of weeks I may have a crack at linking to the granny2.dll in my nwn2 folder to see what interfaces are opened up. From reading the comments above, it appears gr2 is something like a physics rig in that context, but it may be very different in Demigod. Strangely, at a quick glance I can't see granny2.dll in the bin folder of Demigod. Maybe I've got things completely wrong.
So in response to the OP, the devs are unlikely to release a map editor because gr2 is a proprietary format that they can't give you the license to (it costs ~$10000). Whether this is an impassable barrier or merely a hurdle for the community remains to be seen, but I think someone is going to need to figure out the gr2 import/export problem before we can really get anywhere.
TL;DR: Don't hold your breath for a map editor.