If I understood your question correctly, are you asking if Veil will be effective it applied to the interior of a plastic license plate cover
If so, my answer would be that I would expect Veil to be effective when applied in the that manner, but perhaps not quite as much as if it were applied to the outer layer. Although I haven't specifically test one method versus the other, that's what I would expect.
The difference, though, may be practically inconsequential.
Sorry I couldn't provide a more definitive answer, at this time, to your question as we haven't specifically tested one versus the other.