Nice - a table! I was considering doing that. Good job! @272 issue: ugh. Yeah, I was concerned with compatibility across ports, not within them :)
That region isn't free. I've built this table. The 600-799 range is free, though sandwiched between EDGE types. 900+ range is free.
While doing this, I've discovered that Doom Legacy uses 272 for a FraggleScript starter (inherited from SMMU) and for a sky transfer (inherited from MBF). Both effects are there simultaneously.
I'd recommend using a MAPCOLOR lump grouped with the map lumps, personally. (Also, "COL_START" is nine characters. :p)
Oops. I really didn't intend to initiate a dialog of people telling you how to build your port...sorry about that. However, I'm not sure were we're meeting eye-to-eye, in reference to your comment. The transition to 32-bit color might be obvious, but don't short-change yourself on the importance of it, or, rather the impact. Your implementation is not specifically "the first", but I am believing that it is the most sought-after, if you know what I mean. That gives you a bit of "power", so to speak. The power to basically dictate how 32-bit color will be setup and configured, and used, in all other ports from now on.
Though I have to admit that the changes I made are on the simpler side and the transition to more colors was an obvious step. Nonetheless the "old" palette based look will never be phased out as it adds to Doom's unique character.
The only interesting code that has been produced is the screen scaling/stretching code in SSE which is unfinished as of yet.
I am asking you to use that power, to build it in a good, compatible, straight-forward way. And that suggests: Unique linedefs, and control sectors - the Boom way. Special map lumps, and the like, require special map editor support, which would be nice...but not as readily adopted.
At any rate, I put that out there, for you, but, again, I do not want to control your port either. But, please do recognize that there's a lot at stake. It's that important. Do it right, man! :)