In related news, Sinn Fein called for a Border Poll (referendum on Irish reunification) in Northern Ireland yesterday, while Plaid Cymru called for a Welsh Independence vote if Scotland leaves the UK. Brexit's going well - and it technically hasn't begun yet...
Of course, you nincompoop, Wales voted Leave, so they have no grounds whatsoever.
I think it would be reasonable argument that, if Scotland leaves, the Union has changed sufficiently that Wales would be justified in requesting a referendum on independence also, Brexit or no Brexit (would have been true in 2014 also).
It would fail, though.