gothitelle has shadow tag but it is currently unreleased.I'm pretty sure the goth thing has Shadow Tag and in NU.Anything with Shadow tag will come out of NU, always. That said, the main problem that using encore+disable will be be dewgong's low speed and average bulk. Your opponent will have hit you atleast twice before this can be pulled off.Dewgong (I'll give you a few minutes to stop laughing :P) has access to Encore + Disable. While this would likely be better in DP where Encore last longer, would Shadow Tag as an ability be enough to raise it out of NU? The idea is that the opponent is forced to keep using their Disabled move, and thus Struggle to death with no escape.
However, if dewgong does get shadow tag, the deadliest combination would be perish song + shadow tag.
But yeah, it'd be a better Perish-trapper. Seel might work with Eviolite, but even if it does work and your opponent doesn't switch you'll have to take two attacks which Ice types can't take unless you predict flawlessly.