If you are going to try puttying over the screw I would wait for a day or so to make sure it sealed the leak, then sand, clean and whatever is required to meet the directions on the sealer and hope for the best. In my book you have absolutly nothing to lose at this point.

 

Ive repaired holes in my aux tanks before. What I did was to get a bolt and a couple washers and cover the lot in permatex #2 then run the bolt thru the hole with a washer on each side and tighten good. Hasnt leaked a drop. Carriage bolt with the round side facing out? that would be even better.
problem with a diesel fuel tank is you cant chemically patch it because of the fuel, and welding it with a plastic welder doesnt work well either. So youre left with a mechanical solution.
You might even do this.. get a bulkhead fitting from mcmaster-carr with viton seals. the diesel resistant plastic ones are nylon, HDPE, kynar. Then install a pipe plug in the open end. Could make this into some kind of drain?
Its illegal to have a gravity feed setup and anything that can loosen up and drop fuel.. so as far as the legal aspect Id stick with the nut and bolt idea.

 

Drop in a bulkhead fitting and make yourself a bottom drain.

You could try a "boiler plug" I have used them to seal holes in tanks and some have expansion bolts if the hole is too big.

There are also expansion plugs that the diameter expands to seal the hole, they have some for when your oil pan drain has stripped out, it goes in the hole and expands.
You might have to open the hole up to maybe 1/2" to fit the plug.
Jim