In the US, the rule is pretty clear

*6.6.1 Each sailplane is prohibited from carrying any instrument
which:
* * * * * * * ** Permits flight without reference to the ground.

I'm not on the Rules Committee but it's arguable whether a T&B that's
disconnected from power (or the on-off switch) permits flight without
reference to the ground.

This is a brilliant reading of the rules. Yes, you could certainly
argue that if disabled to the CD's satisfaction, the instrument no
longer permits flight without reference to the ground, and thus may
still be carried. It's just ballast. If you convince the CD, you don't
need a waiver.

It still would be good to talk to the CD about this ahead of time,
rather than 9 am on the first day of the contest. And I am sure there
is precedent for how to handle this issue, since lots of gliders have
installed T&Bs.

BTW, though I am on the rules commitee, only Hank Nixon or Ken
Sorenson speak for the committee -- my posts are only personal
opinions.

John Cochrane