Anything we can get on non-GW stuff (or at least non-core stuff, which all plays quite similarly) would definitely be helpful
Most of what we need is concrete case studies of "must be able to create an army like this for game system X", or in the case of some of GW's special rules then it is the race rather than the game system that complicates things.
I remember 40K 4th Edition (and probably 3rd) having experience as an "appendix", but I didn't realise they'd dropped it from the core rules. I guess it lets them sell things like Planetstrike with campaign/experience rules instead
Purchasable XP would be the alternative way of handling it, but seems quick hackish IMO. The only problem I can see with having it as a separate value is that some game systems may allow XP to be "spent" on skills (IIRC your Mordheim characters could get a skill every #XP). That'll need two "purchasing" systems running side-by-side. Not impossible, but an extra thing to take in to account.
A "promotion" option is all well and good, but it'd generally still keep them being called whatever their Henchman type was. I can't remember at the moment whether the upgrade actually changed their type or not.
We probably could have a secondary count such as "rating", but we'd need to plan how to make it sufficiently flexible. I can't remember whether Gorkamorka worked the same way. I had a vague recollection that it had some "teef" value that counted cost plus left over teef, or something.
In terms of the historical battle example, I'd almost be inclined to to treat that as either a special upgrade that doubles their points value (or whatever) or else a separate "experienced X" unit type (since presumably there can be other differences, such as stat lines in some cases). If nothing else then it's similar to what Warhammer does in balancing one-sided campaign battles by giving the smaller side bonuses. In general that's out of the scope of the roster creation.
I'm not sure what you mean by "composition systems", though