Well... it wasn't migration! I migrated and still had the same issue. After much consternation, it was found that one of the VM ports had somehow got prime line assigned. Once I removed that, the MW worked fine. And, FWIW... even without migrating, it seems as though everything was working. (I had already changed the trunks to 4 digit, and the stations all matched the ECS)

Thanks, Keighlar and JB for your (always!) spot-on input!

JB- is it only the MW feature codes that can't be passed by a regular station port? I'm pretty sure that I've used the 5xx codes from a digital station to activate certain features that didn't appear on a designated key.