Hi Attila,
The changes required to use iolib instead of usocket are very tiny. That's great. If only there was some widely-supported, nice way of parameterised module loading. I don't want to simply incorporate your patch as it is -- that'd break windows support. One approach would be a feature :postmodern-use-iolib that people can push onto *features* before loading Postmodern, and conditionally replaces references to usocket with iolib. Would that work for you?
As for replacing force-output with finish-output, why did you do that? It seems like it might slow some things down, and unless I am missing something, finish-output does the job.
Best, Marijn