I think the simasimac-like pattern of a ROM-upgraded SE/30 on power-up is because if this:
Those "modern" ROMs (for the IIfx, IIci, IIsi) were never meant to be used on a compact: they expect either the RBV chip (ci, si) and/or a NuBus card. These video architectures seem to need being enabled by the CPU in order to show dedicated VRAM (or stolen DRAM) contents -- that will happen after memory test, before that video output will be halted anyway, thus the ROM won't bother clearing the VRAM contents.
But the video circuitry of the SE/30 is rather dumb: just a few counters, PALs and multiplexors. This surprisingly simple circuit made from off-the-shelf components will show VRAM contents on the monitor as soon as it's powered -- even with a crashed/halted/dead CPU! The real simasimac is when the CPU can't start and no VRAM initialization is ever made.
It seems that the common ROM on the IIx, IIcx and SE/30 tries to deal with the compact video circuitry, if present: filling the screen on the dedicated VRAM with a dot pattern, before the (possibly long) memory test. But other ROMs won't do that and the compact's video circuits will show the VRAM's semi-random contents st power-up. If you turn off briefly (1-2 sec) a ROM-upgraded SE/30 and then turn it on again, screen will show a more-or-less distorted view of the last picture displayed...
2xSE/30, Colour Classic, Mac II, IIx, 3xIIsi, LC, LC475, LC575, Power Quadra 700, Quadra 840AV, 2x5200, 7200/90, 7500/132, 7500/233, 7600 G3/450, PowerBook G3 Bronze, iMac DV SE, MacMini G4, MacMini Core2Duo, iMac 21.5" i5... and SGI Indy R5K