My experience with most polyurethanes is that very little can affect them in a serious way. Solvents that destroy plastics have very little effect. I don’t know Ninjaflex or other TPEs in this context, but my instinct would be that *if* you can get good adhesion to the copper, it will resist etching. It should be much, much less active in interacting with the acid than the copper is. However, what I would be primarily concerned about is whether and how you can remove it from the board after etching. If you can find a way– which might be as simple as melting it off — i would go ahead and try, with excellent ventillation. Even the acid fumes alone require that.