Would convenience checks written in conjunction with a credit card be considered an EFT?

What do you mean written in conjunction with a credit card?

If a convenience check is written and the paper check is processed as a paper item it is not an EFT.

We're trying to determine if a convenience check written off of a credit card account would be disputable as "unauthorized" under Reg Z if lost/stolen.

If it an access device card/check/electronic used to access credit it is Reg Z if I understand you correctly