You could put the poly on and then go over the signature with a white wash of some kind so your signature is lighter than the surrounding wood…..or fill it in with black paint after poly and wipe off the top surface.

 

you could try on a small piece of scrap where you might have tried the stain first and see if it gets on fire. I would say it won't as long as you do not put the burning pen too long on the wood. Let us know what you decided to do.

 

it won't go on fire.
you don't have an open fire, only an electric heated element.

 

I would just finish as planned, and after the poly has cured wipe a bit of white furniture polish into the signature. A light colored pencil will work just as well.

 

Thanks for the quick responses as usual!

Pintodeluxe and dhazelton - Wanted the signature to stay dark since the wood, although stained is still relatively light.

Decided just to try it out since it isn't a real obvious spot. Turned out just fine and looks a lot better now. Thanks for confirming that it wouldn't turn into a fireball haha

 

Don't do as I do do as I say "Don't be in a hurry." LOL!