Offline indexing with multiple shards

Sorry to come back to this...
I took the time and evaluated other threads:

All have the general advise "Don't do it" but none has an explicit "You can't do it because of x".
The only thing mentioned is the routing of documents via document id. But since you providing custom routing values and custom routing functions i see no problem with it.

So we are now in an unfortunate situation that our devs saying "Yeah, we have a working prototype for ES offline indexing" and the ES experts saying "Nope, its not possible".

So is there something we miss ?
Any insights appreciated!
Johannes