I thought I'd update people to let them know that I finally got around to testing it and was able to get my Focus Enhancements EtherLAN SC-T working in order to connect to the general internet on my Mac Plus. I'm using MacTCP 2.0.6 and MacWeb 2.0c with Internet Config 1.4 in System 7.0 (the network stream is being fed from my old iBook G4 over a straight ethernet cable).
It's surprisingly snappy for an 8MHz machine with just 4MB of RAM! I have MacWeb set to not auto-load images, and I'm able to visit and search MacintoshGarden.org quite well, as well as Frog Find. I have to use the Garden's ftp links to download things, but other than that, it's quite nice. And Joshua Stein's Wikipedia viewer works nicely as well (though articles don't always load in-full, not sure what's causing that; I'll have to try increasing the memory allotment as he suggests).
The EtherLAN is running fine at 9V, center-positive.