I don't think Blizzard would do this, at least not like this. I think the audience for anything that provides no loot or XP would be too small for the development time.
If the idea was tweaked so that it did drop loot at the end. e.g. only the first time it's run, chances of legendaries based on options chosen.
I don't see a way that XP could be given though, as I agree it would be a way to find the most optimal monsters and map, and that would then be the only place to farm XP.
I think the static rift thing is out too for user created custom rifts, as the storage space required to store locations of mobs and pylons, etc. would be too great. I'm pretty sure the game spawns them dynamically as you explore the map...maybe even the map works that way?
I could see Blizzard adding custom rifts that they create (automatically) and then people being able to run them, with loot and XP. It may be feasible to do static maps too then...but maybe not.
Hmm. I think I would opt for something a bit simpler. I know there are guys that really get into this stuff, but, I'm not one of them. There are guys that really get into this kind of stuff. Look at all the cool things on the SCII Arcade. They make all kinds of stuff with that engine. But, again, this all seem very complicated and something I'm not inclined to do myself.
What if you could just save the Grifts spawns? After completing a greater rift, Urshi would give you the option to save the spawn. I agree that XP shouldn't be included in these. But, if you could save the maps after you completed it, I might be inclined to redo a Grift. Like a do over. Say you had Grift you completed with 10 seconds over the timer. I would keep this spawn just to see if there was something I could have done better. This would be easier to implement because the UI would be just a list of the 10 or 12 Grift spawns you are allowed to save. Again no XP, since this would just be practice. But I like the concept of practice. Currently D3 is a game of persistence. If you play enough, you will overcome RNG because you are exposing yourself to larger sample size of the loot tables and that item drop you need for your build will happen eventually. And if you play enough you will gain paragon that will add skill points that will make your character more powerful. So rather than persistence being the main driver to the game, some amount of skill could be added. I'm not stating that skill isn't required in the game currently. But learning to deal with certain mob types would be helpful if you could develop some skills by practicing against a saved spawn could help develop some strategies and tactics. Current skills is more or less hand eye coordination and quick decision making. If you could redo a grift, you could definitively know whether your decisions where optimal because you could actually test out "Plan B".
Bit of a bump. Clearly a custom rift idea won't happen in 2.4, but hopefully in the future maybe something similar. The static rifts proposed to occur in 2.4 are at least a step in the right direction for a more healthy 'competitive' scene, and hopefully the devs are just 'testing the water' to see if people will enjoy some type of static rift.