Scratch that last bit, after a little extra fiddling, and moving my adv3lite directory INTO the libs folder of TADS3 (rather than externally where it was) I can now get t3make.exe to build it.
So that gets me (almost) back to where I was before I decided to convert to WebUI!
It would be nice if I knew how to get htmltads.exe/tadsweb.exe to open my .t3 file and host it on a specific port/etc, rather than the way I do it now - some JS tells me what URL is being used, and I simply copy that into my browser.