Try white spirit, soak it and wipe away thoroughly (may need to do this several times). It cleaned up the damage on my boat when I had a serious diesel leak. And the varnish looked fine afterwards.
Dont know about ply, but when I have to degrease steel, I massage handcleaner , that gel like stuff one washes later off with water, into the steel and wash it off.
Dont see why it should not work as wel or better with diesel.
If the diesel is still fluid on surface with general soak-in ..... then repeated leaving fullers earth spread in a thin layer on it will 'draw' out the main bulk of it ....
Fuller Earth is what is the basis of Cat-Litter .... it soaks up just about anything ....
So spread it on .... (powder form is best) .... leave it for a while till it changes colour ... trowel it of, spread more ...
You will probably have to do it quite a few times. For final lifting of stubborn - then the spirit as someone else suggested to dilute / lift the diesel and fullers earth again on top.
You may not have ply suitable for a door at finish - but it is a waste of ply to just bin it .... other side is probably fine ? Then use it to make other bits and pieces that you only see one face ?? Cockpit table ?? etc.
On my (new to me) Moody33, the fuel tank had been leaking and all the ply boards covering the tank area were soaked in diese. I bit the bullet and made new ones with good quality exterior hardwood. And I will give them a couple of coats of varnish to prevent it happening again and to make them look good.
PS - I replaced the complete fuel tank.