You should certainly be able to change tapenames or timecodes when in 23.976p NTSC.
The MTC is TC30 because there is no such thing as an NTSC tape with something other than a TC30. Meaning that any film-to-NTSC-video process would mean source tapes have TC30, and if you were to play out a sequence to an NTSC deck (which was always assumed at the time), that deck would have a TC30 as MTC.
As said, Avid lets you calculate, track and display a TC24 anyway. It’s just that base TC in NTSC is always 30.
In 23.976/1080p projects, the TC base is TC24.