You might want to look at custom section in ACS classic 3.x if you have access to a running version. I looked at custom sections in ACS classic 3.3, and I couldn't manage to add a custom section. If it doesn't work in ACS classic, then it might not be worth porting. But maybe it does work and I didn't understand the spec.
If anybody knows anything about how custom sections are supposed to work, I'd be keen to hear about it. Others have expressed bafflement/curiosity on the topic.
-Alex