To me, the rule (which is indeed flexible and probably more like a guideline) for hyphenating off limits is probably the same as for any other compound adjective. If it appears before the noun, hyphenate. If it appears after the noun, the hyphen is not required unless it's needed for clarity. That last part is a judgment call.
The off-limits snacks tempted her. The snacks were off limits.
The sweater hand knit by Mrs. Smith itched. The hand-knit sweater itched.
I would, of course, bow to my publisher's house style sheet, if I had a publisher. If they want it as off^limits, that's what I'll give them.
Maryn, short of 1000 words for the day