Thanks for the comments so far.

Yeah, i thought about the trailing knobby, and thought it might be worth creating a tiny platform for that. Something like roller-blade wheels that are relatively cheap and have low rolling resistance, as well as holding up well. All I'd need is a simple axle and mounting frame to tie to the rear wheel.

I thought about removing the front wheel and using a fork mount, too. I hadn't thought of zip-tying the front wheel to the frame though! That's actually a really great idea, but it does require carrying zip ties with me. At least the plastic can be recycled.

It may not prove to be worth it as a short-distance idea. I think having something like this would be worth it enough simply to tow a bike to the shop with another bike. I do think it would be worth it for me for a longer-distance trip, and may actually entice me to consider biking to farther-away distances for long-weekend vacations.

I suppose it is definitely something I'll need to try out to see if the road bike plus drag resistance and weight of towed bike actually proves more efficient than mountain bike alone.

Always good to think of different options. I'd like to move to a more car-light sensibility, so I'm trying to think of every option possible.