could you use checkboxes for display? I'm no sure if you can style them, but I think you can access them in JS, and that should result in having basic "pixels" which you can use to draw anything.
One other way would be to use normal text fields and leveraging custom fonts. I think there are an enormous potential with fonts in the realm of pdf hacking. I think there is also a story of past vuln on pdf.js because fonts were evaluated outside the sandbox.