Ignorant question: I haven't looked at your code, nor have I worked with SVG outside of Inkscape and such, but it sounds like you are using saved SVG files? Is that correct? Since SVG is an XML format, would loading the SVG map elements via code (classes?) work? You wouldn't have to load any local image files. Like I said, ignorant question.
I haven't worked on my 3D virtual tabletop, mostly because Bradley Shepherd's Battle Map Studio looks so good. But if SVG isn't practical for Shamat, I might try to create a utility that imports the Shamat format and exports printable SVG maps, via Node.js. My "optimism" is only based on the fact that Inkscape can be installed in Node.js and run via command line verbs. Folks should only read this is they want to be REALLY bored lol:
https://how-to.fandom.com/wiki/How_t.../List_of_verbs Again, I might be ignorant, this time in relation to CORS and Node.js Inkscape. My motivation would be the ability to export multiple-page PDFs to print and tape or otherwise connect together.