you could certainly extend the unused transactional support to do a chandy-lamport thing, with the difference that when you run out of space isolated cache versions, you just can't speculate any more.

it would be a lot of machinery

you could also do latency hiding with smt instead of trying to fight it head-on with speculation. ultimately probably more productive, but either the compiler or the programming model or the user has to expose that concurrency.

If all speculation used a separate cache that is only committed on retirement, that should keep the side channels at bay.