I've not tried - have had plenty of opportunities whilst the engine was out
I would suggest it is possible to separate the gearbox without removing the engine but you won't be that far off. I'm assuming PG1?
Get the car fairly high up. You will need to pull the driveshafts (so drain the gearbox oil first), support the sump on a block of wood on a jack, take off the steady bar, Most likely take of the exhaust, or at least split it where the down pipe joins the rest, disconnect the gearbox mount, probably take off the clutch slave and speedo drive plus gear change cables (or rods).
I think the engine/box can then rotate around the RHS mount sufficiently to separate the 'box from the engine.
To be honest, by the time you've got that far you are only a few harness connectors and two RHS engine mount bolts away from removing the engine anyway. I lower the sump onto a dolly, remove the mount bolts and then lift the car high enough to roll the whole engine/box assembly out - much easier access for re-aligning the box input shaft back into the new clutch then - might be difficult to get them aligned if the engine is still hanging diagonally down...
Edit: Oh yes, and draining the coolant - I suppose that is a reason to leave the engine in! Undoing the main for/aft coolant pipe supports would give enough slack to rotate the engine down...
Good luck!