One old trick that might work here is that some colors – like a light pale blue – print fine on paper but don’t show up when photocopying/scanning/etc. (think security printing on paper checks)
So… You might be able to print the vector art out and have folks sign that with black ink, then when scanned in GFUI the only thing that shows up is the black.
If that works, and you own or have access to an inkjet printer that has a pass-through for thick materials (some Epson printers have this), you could print the vector in the unscannable color directly onto the work material’s masking paper. Then sign that in black, register in GFUI and go! (Printing registration marks in black would help here, too)
I will most likely be trying things like this when I get my (got the “it’s ready” email, waiting on shipping) and will report back if no-one tries it in the meantime.