maybe you can implement it using MSX-USB BIOS?
Please also see my post here: https://www.msx.org/forum/msx-talk/openmsx/networking-suppor... and the links to other posts about it. There Wouter is also saying:
I agree having DenyoNet and/or ObsoNet support in openMSX would be very nice. Though I'm afraid I don't have time to work on it myself anytime soon. Help would be greatly appreciated.
I once looked into porting the ObsoNet emulation from blueMSX to openMSX. At first sight porting seemed very straight forward. The biggest problem was the actual sending/receiving of ethernet frames. IIRC blueMSX uses some windows specific library to do this, while in openMSX we prefer to use cross-platform libraries.
Though I'm sure such cross-platform libraries exist. So any volunteers to help?
That's for one thing.