well, there are a number of possibilities. the most simple 're-wiring' would be to use "jack", for which pd has native support.
ever used jack on windoze or tried to synchronize pd with jack-transport? there's no rewire part of pd, though ...
and there are objects to use pd as a VST-plugin.
ever tried to use it? if so, you wouldn't take this as argument ... maybe it works, but it's more a proof of concept than a solution ... it's usability sucks ...
it's not really a question of free and proprietary, but a question of usability ...
cheers ... tim