Sooner or later the RTC will try, whether failing or succeeding, especially if you got a female RTC that grow bigger and faster IME. If you get a male, it is less of a chance but still a risk I personally would not take, even though a 30" koi of normal proportions should be pretty bulky in circumference.
If you could remove the RTC or the koi later if problems arise, I think it might be tried with close supervision.
The point being, if a koi fits in an RTC mouth even partially, head first or tail first, it would be in danger.