George,
Yes, in principal you can do this with STACK. STACK uses the "moodle XML" format to import and export questions. There are some sample questions here: https://github.com/maths/moodle-qtype_stack/tree/master/samplequestions
The form does have some sanitation checking of user's input, which is often a help. Pure XML does not have this, but does have other advantages, e.g. easy cut and paste between questions. It has long been on my list to create a text-based mark up language for STACK, which abstracts as much as possible away so that over the longer term we can move closer to being able to share questions between platforms. We are a long way from this, but it would be a start.
This system has such a markup language (https://sourceforge.net/projects/aimmath/files/latest/download?source=directory) and WebWork does, although it is very tied to the underlying Perl language.
If you have any success with this, or are able to engineer an alternative text based language, then I'd love to hear about it.
Chris