This is something I have been playing with over the last couple of years, in my case, to make a pdf of just the activities in a book (Project-like), one per page. The xsl isn't difficult, and I have a good starting point for that, which I would be happy to share (although I probably need to update it a bit before it would be useful to anyone else, or with the most recent updates to PreTeXt).
Once you have the custom xsl, it is very easy to use it in the CLI: just add a new target and set the <xsl> tag to point to the custom xsl. With the subset build stuff, you should be able to limit to just a chapter.
Another option that might be worth looking into is the <list-of> mechanism that has recently (within the last two years, I think) been implemented. You can get a list of all the theorems from a particular scope. I don't know if this is useful in pdf though. Search the documentation or maybe sample article?
Another consideration we need to be careful with is numbering. I assume you would like to refer to the theorems by their proper PreTeXt given numbers. This would mean processing the whole document. Would building a pdf of just the theorems (or whatever element you wanted) for the whole book, and then printing just the pages you need for a given chapter, be sufficient?
All that said, I would really like to develop a "course materials" suite of targets that would be useful for your application and others. So if you are interested in working on this with me, let me know.