I would press the sales folks to declare yay or nay on the specific routes you are interested in flying, though it would be no guarantee they remain offered over time. This morning, I checked a few INTL routes and did not see any. I don't know if they still offer on some or not.
I press the salesperson for more info on this matter, and the answer I got was that Instant Upgrades are pulled from D and I buckets, and not C. I specifically inquired about routes such as CLT-FRA and was told they could definitely see much availability in the near and distant future on their end.
If this is so, then TATL upgrades shouldn't be that difficult to score, since D an I fares seem to be quite plentiful on those routes...