what would cause low oil pressure at idle in a healthy 5 litre

not enough oil?

Check for a blocked filter.
Also there is a chance there could be a damaed oil seal somewhere.
but I would be checking the bypass valve and the pump first.

I would use a mechanical gauge before assuming anything.

Some people cant afford them.

If pressure is going down to the point the oil light comes on there is a worry. But if its just going low I dont see a worry. My I6 when at idle sits just above the red. When you rev the engine it goes up to 7 and stays there.
And she is happy to hit 6500 rpm in 1 second.

at idle beeper comes on is it possible to remove sump with motor still in place

Unfortunatly not. From what I remember of the engines a few of the bolts have the K-Frame in the way. You could try to unbolt the engine mounts ( engine wont move anywhere) Then gently jack the engine using the front harmonic balancer up a few inches. Then you may be able tog et the sump off. Once sumps off let the engine back down on to the mounts and lay under there admiring your bottom end. While the sumps off drain dont tip the oil out and look for build up of a grey sludge in the bottom inline with the crank bearings. If you have build up of what feels like a putty then the bareings are worn out and will be letting alot of oil out in the bottom end causing massive pressure loss. If thats the case you will need to remove engine from car and rebuild bottom end. (not hard at all)
Be sure to check oil pump seals to. they may have perished and is leaking back into sump without going around the engine.