It worked on my Titanic player type which is currently in use on Bones4's server.
Huh. Can't tell what it's measured in, 40 is about the lowest number you can notice. Guess I was trying seconds earlier and it didn't work. Regardless, jumpDelay isn't what I thought it was, and only causes a delay between one jump and another as opposed to landing and jumping. Could still use it for this, but it'd be be pretty awkward to use, and you'd have to tune it for every player so it's not a great option.
Age of Time players interestingly enough have a recoveryDelay parameter which sets a delay between impacts and jumping again. Not exactly what we need but it would be useful in some circumstances if it were ported over. They also seem to have a delay between landing after using their grappling hook and jumping, but I'm not sure how that's handled, and experimentation with them in general has caused wildly varying results because I don't know all the details of how this stuff works.
If I were to guess at the nature of the issue, I'd say the game's method to detect if the player was on the ground occasionally forgets up for a single iteration, so some very brief delay between the engine detecting they're on the ground and actually allowing them to jump would probably take care of the problem.