I'm helping out a friend with a stay sail arrangement I haven't seen before.

The previous owner has replaced the hanks on an old No 4 sail with a sleeve and an inbuilt strop. The strop is raised via an old spinnaker topping lift sheave(I think), converted to 2:1, and back to a jammer and winch on the cock pit. There is no separate halyard and the system relies on the deck winch to get sufficient tension on strop. Luff tension is provided on the sail by lashing the head and tack the loops of the strop. Seems like a lot of monkey business and not very pretty. The tack attachment point on deck the boat is solid.

Has anyone had any experience of this stet up before, would you expect it to work in anger ?

You can see what the arrangement looks like in the photo below of the head and tack.

Sounds dubious as you say but more information is required in order to possibly answer more fully.
A spinnaker pole topping lift installation is not usually designed for much load - as regards location in the mast and exit pulley size and fastenings - only designed for the kite pole topping lift loads which are usually not much.
What is the yacht type, is it masthead rigged or fractional, single or double spreaders, are they swept back or in line, where does the kite pole topping lift exit the mast - so close to under one of the spreaders or in the middle of a panel, are there any aft lowers to the bottom spreaders or any checkstay runners to the spreaders?
Why is the no4 wanting to be set as a staysail? Assume for light - moderate wind reaching extra sail to set along with the no1 geona? What is the sails area compared to the foretriangle area?
If your friend has a ~30fter with masthead rig single spreaders and stout mast and aft lowers (and so probably fwd lowers as well), and the kite topping lift is located just under the spreaders and is a solidly secured exit pulley, it could work but you don't want to have the rig compromised or the insurance violated.