I can now confirm that this is the problem (or at least the major one)
using the instructions at Compile Kernel on RPI2 from RBP2-sources-osmc - #2 by DBMandrake
I built a new kernel using 4.2.3 (I tried the -23 version in jessie-devel, also was missing br_netfiilter)
and changed the config to add br_netfiilter (and other potentially needed modules, just in case)
A few reboots, to get the new kernel into place as the booting one, and I did a modprobe br_netfilter, and behold tethering is working… I did have to manually set up a box I plugged in, but once I got that box configured with IP/gateway/DNS/etc, it worked great… it was able to access my network and the internet, via the wireless connection on the ATV.
So, can this PLEASE be fixed for the next image (or kernel update?) release? Right now, it’s certainly broken on the ATV, it is likely broken on the RP (based on the post I link to above), unsure of where else. OSMC offers tethering, so this is an actual broken feature, not a new request.