Following Gaia's tips, I installed Second_Life_3-0-1-238249 mesh test client she linked to earlier AND added vertex groups mFootLeft & mFootRight to the avatar.blend file I'd been trying to use...and YEP, that did the trick. The test client was happy, the calculate button stayed put and my rigged mesh uploaded and functioned just fine. SO relieved!
THANK YOU Gaia and all!!! This has been quite the learning experience and I still have so far to go.
Edit 8/13: My working .dae file fails (the vanishing Calculate button) on the new beta SL Viewer 3 (build 238157) which was just officially released by LL, but that's probably just because Viewer 3's build is earlier than the one I tested with above (which was 238249). So, I guess I've just gotta wait a while for the live version to catch up before I can upload my rigged meshes.