@snailplane That link does suggest that ます has/had a 連体形. Relative clauses (which is the only place you would employ 連体形) definitely does not permit ます on verbs -- why this is the case I am not entirely sure, especially with ます having a historical 連体形. This sounds like an interesting question that should perhaps be forked off.
I've also found an answer on 知恵袋 suggesting it does, as in the examples:
However, Darius Jahandarie appears to suggest it might be "historical" rather than part of the modern language. If the 連体形 only appears in relative clauses and
ます is considered ungrammatical in this position, doesn't that mean
ます no longer has a 連体形?
It seems like one of these pieces of information must be incorrect. How can I make sense of this conflicting information?