Think of how many people are interested in building content for AH (not many). Even less would be interested in modifying code.
HiTech
It depends on how many SW developers play AH and how they are involved in FOSS...
For example:
makeahmap program that most of SEA/AvA maps are now built with. What would be with community content if this program wasn't available?
I can tell for myself that as long as I use some tool a lot I usually find some bugs and fix them. Also I'm not typical sw developer.
So don't underestimate the power of the FOSS community, especially that you see real examples of this.EditI think the more correct question is how big is the effort of open-sourcing some of the code while keeping closed source core + publishing all building testing procedures of partially closed/open source.
And what is the expected contribution you can receive for this effort. You can probably gain much more if you publish the code under non-open source license - for example stuff like CC BY-NC-ND that protect your rights but gives the community an ability to improve the code.
Since:
(a) you still have your copy-rights protected
(b) it would be much simpler as you don't need to change the build/test infrastructure.
(c) the community can actually help