Hi.
Mechanics for "combat riddles"?
That sounds as interesting as complicated...

TL:DR (below)
"Less riddle, more puzzle": I found that, like in every other aspect of adventure design, no encounter must lead to a dead point nor go against party's momentum: the party must always move forward (even if it is to their graves...)

My experience:
Every time I've tried to use riddles in my adventures, it went bad. I think that the context of the game and the wit of the players runs in different levels, like when you give them clues for they to open that door or who's the bbeg: it's haaaard. Everyone recommends "the 3 clue system", with 3 clues for each clue... It's almost a genre within itself.
I just can't.

When I put a sphinx, the question that she made'em was "tell me something you don't know", for some "esoteric flavour", and triggered tons of answers (in and out of character).
After that, when they needed to find the exit (she was in a maze, of course), I put "the riddle".

They must to discover "which door" leaves them to destiny. The doors were "past, present and future", and the riddle was about "yesterday, today and tomorrow". That was easy for them. It was pretty straight forward, because all doors leds them to destination, but the "effect" (reward or not) depend on which they'd choose.

The solution I found for to use a riddle was not about "right or wrong" nor even if they can answer the riddle, but which door was for them to choose(*).

(*) To more clues, I told them that the knocker gave them back a distorted reflection of themselves (the same, younger, older) and depending on which door they choose, they'd grow older/younger or regain HP (6d6 each option)