I cooked the PCI ethernet card that I planned on using with my Pippin with PCI extenderand ROM upgrade so I'm temporarily looking for more modern drivers for the product in the subject line (two names; same device). The most recent driver that I have is Etherlan SC 1.4.8 from the Mac Driver Museum. Please let me know if you can supply anything later or have any knowledge about how well these boxes work with Open Transport and System 7.5.x.
I recall briefly playing with a Focus box a few years back. As I recall, the drivers were particularly horrid in that they install a model-specific driver resource into the System file. The device itself is only nominally SCSI compatible with regards to providing a reasonable device identification and responding to standard SCSI commands.
As for using a SCSI-to-Ethernet adapter with OpenTransport, it really tends to vary from vendor to vendor. The old DaynaPORTs are decent boxes, but only support Classic networking. The Asante's might support OT, but tend to be specific about what Macs they support. The best seems to be the the one from Sonic (also OEM'd by Farallon) as they are normal SCSI devices and the drivers do support OT. (I have one of the Sonics on my Duo 280c running 8.1.)