I agree that if possible (and practical) this would be a better approach than porting say contiki. As you say tho, the issue is likely to be around how much free RAM is available once done, specifically for additional applications to be loaded to take advantage of this new IP stack. Having said that, this is only likely to be an issue for the 128K Mac, not the Lisa or 512K Mac - but I personally would like something that worked on the 128K - still perhaps this is just not possible (which is why I originally suggested Contiki, knowing that this has been ported to systems like the C64 - which I refuse to believe is better suited to this than a Mac - even a 128K one).
Unfortunately I am no programmer and so am likely to be of no help in coding or designing anything, although I guess I could be useful as a tester for the Mac stuff (no Lisa unfortunately however).