(Malachi James answers)
I’ll answer my own question for the benefit of others searching for the same issue:
You can indeed add a border to PrintDraft by adding
\def\PageBorder{mywatermark.pdf}
to PrintDraft-mods.tex. Furthermore, you should be able to manipulate your pdf using the same options that can be passed to \XeTeXpdffile
Along with the example here:
\def\PageBorder{mywatermark.pdf scaled 2000 rotated 45}
you should also be able to use xscaled, yscaled, width and height (untested).
The issue I had was caused by my pdf being version 1.5 while PrintDraft will only deal with pdfs up to version 1.4. I was able to adjust the options in my pdf print driver and create a usable pdf for my border.
If your border is a picture file you can also use it, but it takes slightly more work. You must find the ptx-cropmarks.tex file in your Paratext installation folder and edit the line
\global\setbox\b@rder=\hbox{\XeTeXpdffile \PageBorder \relax}%
to read
\global\setbox\b@rder=\hbox{\XeTeXpicfile \PageBorder \relax}%
Which picture formats you can use is determined by your operating system.