TTI has offered the interior ceramic coating as on option for a long time. I have never seen anyone talk about it either as far as failures or successes. Soot/film may build up on the inside enough that it can't rust or peel off like it does on the outside. Or the interior coating does eventually fail like it does on the outside, but the soot covers it up and nobody has really looked under it to find out.

If I had a new set with no coating inside I would not hesitate to flood the openings on both ends as far as I could get at with VHT header paint. Better than nothing at all.