Ukrainian crews of privateer warships attacking Russian cargo ships on the high seas... NATO appears less likely to think outside that box

Not so much NATO as the US alone. As of the 1944 Breton Woods accords, it has undertaken to protect all blue water international shipping on its own dime. The USSR was part of that agreement, but not the RF. If the US declares the RF a "state sponsor of terrorism", then it doesn't trade with that nation - or any of that nation's trading partners (which would crash Germany's economy at a minimum - major reason I don't see that happening) but what happens to US overwatch when a nation is so declared? Is it lifted? If so, just on vessels flagged by that nation (easy to determine), any vessel that's been in one of that nation's ports (harder, but possible with SatInt), or any vessel carrying cargo to/from that nation (seems hard, but I've no idea how hard).

I don't expect Stefan to know (and he mostly doesn't read responses here anyway), but are there any international trade experts who know the ins and outs of the Accords, and how this plays with a declaration of state sponsorship?

The US needs to start thinking about this, if it's not in settled law/treaty. There's this situation - and there seems to be a China-Taiwan one coming along as well.

All that said, I see no reason not to send Abrams tanks — although there are probably reasons they shouldn’t come directly from the US. But it’s worth bearing in mind that, like the T80, it’s turbine-powered and quite fuel hungry. I understand they’re operated in Poland, so availability of maintenance in a bordering country might sidestep any issues of it being only part of a weapons system, depots with skilled repair people being part of the rest.