Few things to take into account..
Dry method gives a greater risk of bubbles or misalignment. I apply vinyl and digital print all the time for my job and I would still think twice before doing this dry.
If you know what pen was used then go buy one.. draw on a smooth surface, spray with acrylic clear and see if it eats it.. if not - your OK.
OR.. if you want to be 100% sure, protect the signature and be able to use the wet method then ring round for a few sign shops that do digital print. They'll be able to run it through a cold laminator for you with choice of finishes.. gloss, satin or matte.
Laminate is real cheap and is a UV protectant for your print and signature.. cost is around $10 per linear metre for materials so you should be able to get this done for a 6 pack id say
