Depending on the maker, it is possible order a custom sized iron that would be small enough to brand onto a pen's surface, though the actual branding process on an object so small is a trick and a half indeed. Unfortunatly, a pen isn't an object with the forgiveness to sand down in case the brand goes awry. To my knowledge, I haven't yet seen any sort of concave iron. If you by chance have access or are lucky to know someone with access to a laser engraver, that'd be the most reliable method for tackling an engraving process so intricate.