I sometimes have another PDF I want included at a certain point. I thought I could do this using XInclude, but I see that XInclude is only for XML, not PDFs.

Is there any way, either in XML markup or perhaps command line, to include a PDF at one point within the generated PDF?

No, not yet: Prince can't load PDF files, it only generates them. One possibility is to generate a PDF with Prince, then use another tool to load the two PDF files and combine them.