And then you call Microsoft on the phone and they issue a reactivation, or whatever they call it, and everything is fine. Unless this works differently with OEM versions of Windows (different, more obtuse support people?) than it does with retail, I can't imagine it's actually a problem.

Don't confuse the technology with the legalities. "The license" is a contractual right that you have legally purchased. If Microsoft's piracy-detecting hair-triggers are firing in a situation where you still have a license per the contact - oem licenses are usually "on this PC" or the like, so toggling UEFI alone shouldn't breach the terms - that should be their problem. You're entitled to your license.