A native Inferno port would certainly be a lot easier, but I think you might be a bit pessimistic about would can fit into a 64K address space machine. The 11/70 certainly managed to run a very respectable V7 Unix supporting 20-30 simultaneous active users in its day, and I wouldn't have thought plan 9 arriving about a decade later, would have been hugely bigger than V7 Unix.
I recall a demo of Plan9 (I think it also included the source) being given by Rob Pike at UNSW which he carried on a 1.44Mb floppy disc. By its open source release in 2002 the distribution was 65MB
The smallest Linux system I have used recently had 256K RAM and 512K flash. A rather stripped down busybox based system, but it did include a full TCP/IP stack and a web server. Thats comparable to a PDP11 except for the limitation on the largest individual process.
Bear in mind that 16 bit executables are smaller, and whilst the 11/70 had a 64Kb address space, physical memory could be somewhat larger, and an individual process could have 128K of memory is using separate instruction and data space.
I am used to thinking of Plan9 as very compact, but I havn't really looked to see if it has grown much since the 80s, and perhaps it is only next to the astronomical expansion of other systems that it still looks small. It would be an interesting exercise to find out.
It would be an interesting thing to try, if only to get a better feel for how compact Plan9 actually is ...
DigbyT