SL-Mark:
The advantage of such a rule is that, if refused and the indicated number of moves has been reached without a capture, it could then be enforced even though the opponent refused the offer.

I don't think advancement to some particular rank indicates enough of an advantage to justify it as a measuring stick in the draw issue. It would create difficulties in verification and possibly allow a temporary advancement. Whatever the number of moves, I think the capture of a piece by either player must be the criteria.
To allow for slow starters, the 50 move rule could start at move 25 and the 35 move rule for smaller boards could start at move 15 for the open variations.
50 moves starting at move 50 and 35 moves starting at move 30 for the volcano variations.