In the previous topic about the Pretendo demo
, we found a couple entropy sources in the NES to set the PRNG's seed. One is readback of uninitialized OAM, which is 27*64=1728 bits of DRAM, but that doesn't work reliably on pre-2C02G PPUs. Another is the analog result of bus conflicts inside the PPU when trying to read back from VRAM while rendering is turned on, which is what Pretendo uses. I don't know which also apply to the Super NES.
Another that will probably apply more cleanly to the Super NES is reading the controller dozens of times per frame. This can be done "in the background" with a scanline interrupt, such as the NES's MMC3 IRQ or the Super NES's VTIME IRQ, with a programmable cycle timer such as the NES's FME-7 IRQ, or by (ab)using the NES DMC as a timer. The latter produces a 4.2 kHz timebase (roughly one tick per 3.8 lines) during which player 1 A and Start can be read in a DMC IRQ handler. Fortunately, autoreading isn't the only way to read a Super NES controller, as bit banging ultraslow ports $4016/$4017 still works the same way as it did on the NES.