I DID IT
Using master branch of XeTeX, and with many syntax error fixes to xetex.web (ok tbh, it didn't take that long), then
$ weave xetex.web && sed '1s/webmac/xewebmac/' -i xetex.tex && xetex xetex.tex
Will probably be uploading that file somewhere so others don't have to go through this.
New post! Partly to host these weaved pdfs, partly to document how to make them.
In addition to XeTeX weave, I've also weaved the pdfs for TeX, eTeX, weave, and tangle.
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