it's likely been case-hardened, so is probably engravable after annealing (and re-hardening thereafter) or, depending on the scope of the job, surface stoning & polishing. i'd ask Doug Turnbull or maybe Mike Merker of Hendersonville, NC. Mike handles and works on a lot of continental doubles and i've done the occasional bit of restoration engraving for him.
Lee:
I have worked (not engraved) on a few over the years, and the steel is very hard. The frame is a special version of their 20g shotgun action. They beefed up several aspects to make it stronger. Areas like the fences, and tang come to mind. They are heat treated very differently from the normal shotgun action too. You can see it on the internal guts when comparing the two side by side. Just make sure Turnbull or Mike are speaking for the double rifles, not the shotgun actions.