Maybe a coin isn't quite the same as a deliberately carved lintel! I think the archaeologists of the future will like your coin. I like to think of that doorway being buried unseen inside Silbury though, very weird.

When the British Israelites were digging for the Ark of the Covenant at Tara, George Bernhard Shaw went up there at night and buried Roman coins for them to find*.

I agree with marking restoration with a date. All the concrete dolmen restorations have OPW** and the date scratched into them (in case someone should think the concrete was original!) More importantly at places like Drumskinny stone circle where whole stones have been replaced they are also marked. But leaving a fecking portal into fairy land is ridiculous!

* I think it was GBS

**Office of Public Works