Gene, I love the man cave concept. I have noodled around before on this type of idea, but for some reason I never considered a simple pop up. I wonder how many CGs would prohibit the second camper? Some do. I could disguise it a little by removing graphics and getting some sort of business sounding name lettered on it. Something nebulous like Network Support Unit 53A. Or Cathy's Cookies. Anything like that. Might be worth a try.
My thought was a small aluminum enclosed cargo trailer. Get one with a side entry door as well as the rear ramp. Put a couple of RV windows in it, some basic lighting and such. Etc etc. The CGs would not balk at that at all. Either way, I like your idea more, if the CGs allow it.
We don't currently bike or motorcycle although we talk about how nice it would be to at least have one bike for running around the CG. For sure, if we were to add a trailer of any type, it would open up a lot of possibilities.
A toad is another possibility, but it doesn't do anything for the man cave idea, except haul more goodies.