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I know this isn't a complete answer but node.js allows for c and c++ add-ons you could then grab the zpaq c++ source and try loading it into node.js as an add on. I haven't worked with c++ enough to help much more but that might be your best bet. Might require some tinkering and troubleshooting but I imagine it's possible
Not sure if this would work for you, but i'm developing a NodeJS wrapper for Uharc. Uharc is not paq, it is bsc. But it is free and it's 10% faster than paq4.
It uses wine to run in Linux and OSX as uharc is closed source and windows only.