I had given up on trying to come up with a crossflash for the LB4M as it's quite old and you can now get the brocade ICX6450 for only a few dollars more on ebay, which has twice the 10gbE, only draws 22 watts, and is near silent.
However
@abq was nice enough to send me an LB4M! And after some exhaustive searching today I found the Extreme Networks/Enterasys C3K122 and the B5K125-48p2
They both have the ASIC we need, the BCM56514. However they also must have a MPC8541 management CPU - The C3K122 has an "MPC85xx CPU", so there's a chance it is indeed the matching 8541. I can't find much info on the CPU in the B5K125, but release notes for it do mention an L2 cache bug in the "MPC85xx processor", so there's a chance there too.
Extreme networks FW is very locked down - if someone here has a support account, please get in touch with me. I have an old contact there that I will reach out to as well, but it's been a while. Again, if you're shopping for a switch to buy there are MUCH more cost effective options, but this would be a nice bit of fresh air for people who already own LB4M's
EDIT: the B5K series is a no go, it's an MPC8378