Stan,
Hot tanks for steel and cast iron use sodium hydroxide (lye) and potassium hydroxide in water. Both are excellent paint removers, however following hot tanking, the parts are rinsed with a blast from a water hose. If they haven't, the parts will have a soapy feel when you touch them with damp fingers. Both sodium and potassium hydroxides are very soluble in water, so they are removed immediately by a final water rinse. Any residue would be unlikely to cause a problem because there would be so little. However, depending on how long it has been since the caustic solution was last changed, you could end up with some oily residue clinging to surfaces. Before painting, it's always a good idea to wipe down the parts with solvent.
The POR15 sounds like it's a 2-part linear polyurethane, so it would probably be a good one to paint the inside of the crankcase with. I am almost certain residual sodium hydroxide in biodiesel won't touch 2-part polyurethane. However, anyone running biodiesel that hasn't been neutralized (a simple final step) is just asking for problems. I expect the injection pump and injector(s) will be the first to act up.
Shipchief, you really ought to have something in there because even the best quality cast iron is porous.
Quinn