There shouldn't be any worries welding the cyl, as mentioned, extend it or strip it down to keep the heat away from the seals. Most of them are welded together in the first place so there shouldn't be any worries cutting/welding it again. I've cut and welded new ends on several without any problems and I'm by no means a pro welder. Of the 3 I've done, I only extended them, I did not fully strip them. I can't even remember now if I cooled them in water after or just let them cool naturally. If I did cool them, it was so I could continue working with them on the project sooner not worrying about the heat transfer.
I see it differently than you as to the market for generic cylinders. Volume wise, I would guess that the market is quite small compared to standard type ends. Due to volume, I don't see a generic cyl being any cheaper than one with ends. Those folks looking for custom ends usually have the ability to fab themself and do as I did and you are considering and cut/weld a stock one to suit.