The cap is locked on a 722.4 and .6 because Mercedes feels that the transmission must be fully HOT before analyzing the fluid level. On the 722.6, they left the stick out entirely because thay felt that if someone other than an MB shop checked it, that the incorrect fluid may be added, They are making it tougher for shops who shouldn't be messing with it,well, to mess with it. In general, they only want qualified mechanics to be in there. The lock on the 722.4 dipstick (yes, the 722.4 HAS a dipstick but the cap is locked) is way different than the tiny one on a 722.6 cap. The dipstick for a 722.6 is a MB tool. To do the level checking correctly the car is connected to SDS or HHT and the ATF temp is monitored using a temp sensor internal to the transmission. The MB tool dipstick is about 2 feet too long for the dipstick tube, it is inserted until it bottoms out. So you can't just buy the special tool dipstick and just leave it in place.
Many owners come in for service and the lock will be broken off on a 722.4. It's no big deal as the dipstick still has a "lock" that the orange colored lock is supposed to keep from being unlocked. On the 722.6 trans, it's a little more important to keep the lock in place, as the lock is the only thing that keeps the cap in place.
The 722.6 ATF question is a good one. I'm not sure. They alluded to the fluid being synthetic when I went to class on it, it's 10-15 bucks a quart, made by Deutsche Shell in Germany.
In answer to your question, I don't know if it is OK to put the 722.6 ATF in any other trans. It's pretty pricey to just try it.
Nor do I know if they feel the 722.6 is that much better of a tranny that it can use a lifetime fill of fluid, or if it's just a real "Super Fluid" that they do this. Probably a combination of the 2.