You don't need a new exporter to create levels with blender. What needs to happen is a lot of work on the rendering end of things in the glest source.
Right now the grid system that the maps (and the characters, and objects) use is also how the world is defined. Each square represents one world unit. This makes it easy to create a map for. It also makes it easy for units to do path finding, and for placement, and targeting.
Because of the simplistic nature of this system, you cannot create heights much more than is allowed currently without stretching the textures out to the point they look bad. As well there is currently no way to deal with cliffs or surfaces that are not walkable, so it is probably just as well.
The other issue is that this grid system makes it very difficult to create units of soldiers that can walk, turn and fight as a single group with the current setup. The problem here is that if you can imagine a grid, where each soldier is placed across the grid in line next to each other, that's fine. OK, now it is easy to see how those soldiers could be placed 90 degrees to their original location, or even 45 degrees given that their spacing would increase. Now try to figure out how to place units in line at anything less 45 degrees but more than 0... still can't see it?
try drawing it on a piece of paper, you'll see what I mean.
One more problem is something that you can see in the game currently. Like how sometimes the characters can walk through the edge of a rock, or how they stand sometimes partly buried in a steep hill. It is because the bases of the characters are mapped 90 degrees from the grid. NOT FROM THE MESH THAT YOU SEE. I know that this is something that almost all games deal with to a certain extent but not nearly as bad as Glest. you can literally lose some of your characters inside objects that should be impenetrable!
I am trying to currently figure out enough C++ to fix some simple things in the game, but the opengl stuff is way, beyond me. I only know how the game currently uses it, not how to fix it.