That's quite a large diameter pipe to get blocked. Have both engines always used 254?
It would be a big coincidence for this to be the cause on both engines, but have you tried changing the carbon seals on the centrifugal breather. If they are leaking, it would allow oil straight into the breather pipe which would create blocking (and lots of smoke) when the oil burned off at the exhaust.
My experience is mainly with 2C1, 2C2. Does the 2B have a calibrated breather hole incorporated into it's oil system (similar to Allison 250)?