I'd love to see this come alive. First thing I'd do would be to re-create The Crescent Hawks' Revenge :-)
As for the game engine, I love what's able with Unity.
For an undertaking like this I'm afraid it's the wrong choice.
While Unity shines at implementing games, it's not so well suited for creating moddable game creation kits.
When you make things editable in a Unity game, you lose much of Unity's built-in power because it prefers to read resources from unity-style resource packages.
Shadowrun Returns is built with Unity and it comes with an editor. While it can do lots of things, it doesn't run stable and the game is still not fully moddable because of the reasons I just mentioned.
I googled a little bit and I'd suggest using the Spring RTS engine.
https://springrts.com/I did not work with it myself, but I think it provides everything that's needed for a rendering engine and it's quite up to date (last commit 3 days ago). It's really a RTS engine, so it should come with most of the very basics that a project like OpenMech requires.
Plus, it provides scripting with LUA.
As a couple of game projects have already been created, I'm pretty sure that some editing appliance is already available as well.
OpenMech would still require custom code for the briefings, mech bay, etc.
Once the engine is done (no matter if Unity or not), the real work starts with generating content like mechs and other models/sprites, scripts, a story, etc.
So, for speeding up creation of any OpenMech campaigns, some common, basic resource set with mech models, buildings, etc would be great.
Just my two cents, please feel free to ignore.
UPDATE: someone started working on MechCommander for SpringRTS already
http://sourceforge.net/p/mwspring/code/HEAD/tree/UPDATE 2: here's the more recent version:
https://github.com/SpringMCLegacy/SpringMCLegacy