Update: I switched from the prayer pot method, to absorptions/rock cake. It's going relatively well.
The only problem now is I have noticed that it steadily -- almost every time -- uses the rock cake after the overload runs out, and has to wait for my HP to rise from 47/48 (sometimes far worse), back to 51. At times it does get it right. Other times it gets it terribly wrong. As an example, I just checked and I'm at 27 HP and haven't dosed with an overload.
Edit: took the advice on prayer flicking for hp, and it seemed to solve the problem with eating the rock cake below 50hp when not potted.
Still had a problem with dying when there were absorption pots in the inventory. But I do think, with my current settings, it would run as long as I need. Just with added, premature, deaths.
Edit #2: I'm not sure that I changed anything ... but I just ran close to two hours flawless.