Would this type of acid treatment have any ablative effect on import stamps?
If the markings were very lightly stamped and if you kept replenishing the rust remover with fresh* solution and if you left the metal in the solution long enough it would erode away the markings. Of course, if you continue the acid bath long enough the entire barreled action would dissolve away. In short, these relatively weak phosphoric acid (or acetic or citric acid) solutions are not a good means to remove markings.
I suppose that stronger acids, if somehow contained so that only the area with markings is in contact with the acid (by coating the balance of the metal surface with some acid resistant material) could be used to etch away the import markings.
* It’s been decades since I took a few semesters of chemistry, which I barely passed, so I am not speaking with confidence. I just
assume that as the acid solution reacts with the rust (and the substrate steel) its efficacy diminishes. In other words I really don’t know much about this subject and readers should regard my comments with skepticism. 😉