The problem with the SIMM expanders is that you're increasing the number of power-sucking chips by a quite a bit in addition to increasing electrical loads on the memory circuits. I wouldn't likely use them outside of a PC since they often have more power to spare and are more tolerant of weird memory configurations. Plus they're really tall so wouldn't fit well in a compact Mac or most of the II series or similar machines with limited RAM clearance. That said, I got a set of 72-pin SIMM-to-168-pin DIMM expanders in a box of random stuff once. Never tried to use them but they may work in old PCs that can use unbuffered EDO DRAM DIMMS.
Since some people are working on remanufacturing SE/30 boards anyway it shouldn't be too difficult to reshuffle some wires to accommodate 72-pin SIMM sockets in place of the 30s. Maybe flip the ROM SIMM 90 degrees and scooch the FPU over a little? Or maybe just use 4 72-pin sockets instead of 8 to keep things mostly in their original orientations? 4 sockets will still yield 128MB.
The paths open up in AI9, but the nice type devolves into simple paths. This is doable, below is only a screen shot. Components and connectors on yours are missing, bitmap overlays? line weights and colors are easily fixed. Which pages line up in what order to left and right of this page?
Three layers showing control signals on one layer with data and address buses on the other two for viewing individually might be helpful?
Made a major purchase that I wanted ever since I first heard about the 550c from the Cult of Mac book years ago -
I picked up a Powerbook 550c with the box and manuals for a grayscale 520 for $350 shipped from Japan.
It had some minor cosmetic damage to the screen from a leaking PRAM battery ("vinegar syndrome"), unfortunately, but otherwise is in pretty great shape! Already upgraded it to 40MB of RAM and the original 750 MB HD works great.