I have tried it a long time back on my .470 which has Southgate ejectors and something didn't work right, but I couldn't remember what happened.
Just tried it again now.
It seems to fire both locks just as the action is about to fully close, then it fires both ejectors when you open the gun.
With a non ejector, you can usually ease the action closed with the triggers held back, and it will gently lower the tumblers, thereby releasing the tension on the vee springs.
With my ejector gun, no matter how carefully you close the action, (with triggers held back) the locks release with a snap just before the action closes.
Like I said, different ejector systems will most likely behave in different ways when doing this.

The thing is that the ejector mechanism and the lock mechanism are designed to operate in a specific sequence, and are timed to operate correctly during normal use of the gun.
Closing an ejector gun with the triggers held back, completely alters the sequence and timing, and who knows what damage could be done.

Bad idea I reckon.