68kMLA

I would give it a vinegar bath. Just the regular 5% from the grocery store, submerge and scrub with a toothbrush. Might need a dental pick or something to scrape any stubborn hard corrosion from component legs. Should need less than 10 minutes in the vinegar; probably closer to 5. I rinse thoroughly with distilled water afterward, then isopropanol to dry it out. You could even bake it to make sure it's fully dry.

does it work?
try an ultrasonic clean

Don't own an ultrasonic, unfortunately, but I was planning on doing the vinegar->distilled->alcohol wash, so thanks for the second opinions. This board "works," but most PDS cards will prevent it from powering on, so I suspect shorts or trace rot. Just this summer, it had seemingly no faults when used with these same cards. I've got every chip in the crustier areas removed, so no electrolyte can survive the wash.

A lot of the pads in the more corroded areas pretty much fell off from minimal force. I imagine this board wasn't actually cleaned at all before the last owner put the new caps on, despite how clean of a job they did. If the gunk wicked into/rotted any vias, then I'm concerned this board might be above my skill level. I can at least rescue the ICs and enjoy the Mac as a normal SE in the meantime.

@Boctor - if PDS cards prevent it turning on, it might be that the 5v rail is a little low and the extra load is pulling the voltage a little bit lower again. There is a trim pot in the PSU for adjusting it (but be super careful around mains - use a plastic tipped screwdriver for trimpots (or carve one from a chopstick)). The 5v on an SE/30 needs to be very close to 5v. With 4.95 to 5.1 really.