MWBC's for Dedicated Circuits?
Login timeouts ate two well crafted posts in a row (throttles sysadmin). Summary with have to do.
After very careful analysis I think AllanJ is only saying that you can't bridge or otherwise cross-connect the load side neutral of a GFCI outlet to the line side neutral. Read that 10 times if you have to, that's my story and I'm sticking to it.
Apparently AllanJ's "code" fails to recognize that what he describes is a not-so-special case of exactly that. True: jumping the load and line side neutrals of a GFCI outlet with a 2 inch jumper just don't work (I'll bet we've all tried it once). AllanJ is describing a "jumper", maybe 10's of meters long, which also sees or SHARES service as an MWBC neutral.
That won't work either. A short is a short is a short of course (unless the short is Mr. Ed er, I mean radio frequency), whether it's across the terminals with a screwdriver or long-pathed in the form of a misused MWBC neutral.
The problem is not that his advice is false: the problem is it's almost trivial. The problem is not that the scenarios he describes would be dangerous: it's just that GFCI's would start popping under any load other than 0.
What AllanJ describes sounds like code translated into practical/trade wisdom for using GFCI's with MWBCs. If the NEC goes so far as to point out that the load and line side neutrals of a GFCI must not be directly or inadvertently connected, I'l bet the rule is far removed from any treatment of MWBC's. But I wouldn't put it _entirely_ beyond the NEC to be redundant.