So far no one has given a clear answer. Since Valve hasn't released any SDK tools, its impossible to preview rigged sub-models without the CS:GO model viewer.
If anything, TF2 has proven just how inattentive people at Valve can get with models, rigging and texture issues and all.
Your best bet is to indicate to the best of your ability (pictures, private developer workshop comments, prototype rigging if you think you know how it will turn out, WORDS, etc.) where and how the items are supposed to fit. Anything to make it as easy to understand, test, and rework the least amount as possible.
Alien Swarm SDK.