What's the current status on major mining pools accepting Lock_time transactions / higher sequence number replacements?
They don't.
No one will accept your transaction until it is final (nLocktime in the past and/or nSequence at maximum).
It seems like a simple incentive problem. What if when a user creating a tx that is locked in the future, could offer a reward for either a node storing the tx until the lock expires (then it will broadcast it). Instead of only having a fee that goes to the miner of the block, an additional fee could go to the node that stores and eventually broadcasts the transaction.
I guess the naive approach would be for someone creating this future tx to create multiple copies of the tx, each with a different destination "rebroadcast fee" address. So certain nodes could advertise "I rebroadcast transactions up to n blocks from now with a fee of n*(fee)". Then the user creating the tx would create a new copy of the tx for each "rebroadcast node" it sends the tx to, with that specific rebroadcast node's fee address.
Maybe this has been discussed somewhere else, but just a thought.