Commercial products basically can't involve out-of-spec operation of chips, so naturally nobody sold upgrades of this type. (Though some companies did seemingly certify 40 MHz 68030s to run at 50 MHz.)
But for someone with some technican's skills, it wouldn't be too hard of a project, I don't think. The hard part is finding and cutting the "C16M" 15.6672 MHz clock signal going to the video circuitry and wiring up another oscillator to it, separating it from the rest of the machine to be overclocked.
Of course, there's a high probability of destroying the SE/30, at least beyond anyone's capability to repair, and it might just turn out that the mod won't work at all.
Also if the GLU chip and other peripherals can't run at 20 MHz, you could try increasing the voltage from 5V to 5.5V or 6V. Obviously you don't want to crank it up too much, but that extra voltage might reduce the propagation delays in the chips that limit their performance to ~16 MHz.