Roguey As a licensed title (ie, another company's problem, lawsuit bait!!), I think it's rather unlikely that Planescape was "under the radar" in the same way that Fallout was.
It was an unusual context, where using the D&D licence was far more trivial than it would usually be.
Interplay got the D&D licence in order to make/publish some major AAA Forgotten Realms games. Bioware came to Interplay with their RTS alpha demo, they talked and Interplay realised that the Biodocs also liked crpgs and were familiar with D&D. Interplay convinced Bioware to make their Forgotten Realms games for them to publish - which became the Baldurs Gate series.
So they'd already published the game for which they obtained the licence, and had their major follow-up sequel in the pipeline, with plans to develop their own dungeon-crawler Icewind Dale game using the same engine.
Very different scenario to where they'd be if they'd licenced D&D specifically for Torment, and entirely consistent with it flying under the radar. They were using a licence that they already had for other projects, where they had a broad right to develop games using that licence for a lengthy period, the licensor and everyone else involved was happy because Baldurs Gate sold well and BG2 was looking good. Torment used the D&D licence in the same way that it used the Infinity Engine - ultra low risk/cost because they're just taking assets (including the licence) that they
already had for far bigger projects, and were pushing out a cheap little slamdunk. D&D didn't care because Planescape wasn't a big property like Forgotten Realms, and BG was giving them a computer game foothold that had been lost since the Gold Box days - they didn't even need their permission, because they already
had the licence, and there was never going to be any problems so long as the 'main projects', i.e. Baldurs Gate series and Icewind Dale, were selling well.
If anything, it would be easier for that to 'fly under the radar' than if they were developing an original IP. In those days, developers - especially large developer/publishers like Interplay - got to
keep their original IPs, so developing a new IP was an opportunity to add a major long-term asset to the business. PS:T was structured, business-wise, as a cheap cash-in. The equivalent of churning out a FPS expansion pack, where it's just 10 more levels of the same engine/gameplay. Really minor stuff, just a small bit of 'value-adding' to milk the licence for a few extra bucks at no risk. It just so happened that Interplay/Black Isle was chock full of talent, and the cheap cash-in ended up in the lap of MCA and a really talented team.