Good idea. (I'm talking to you, scientists.) In the meantime, I dissolve a tablespoon or so of this in a small pump sprayer to erase waterline stains with minimal effort:
https://www.amazon.com/Savogran-10501-Wood-Bleach-12/dp/B000VBGH82
Basically, the methods here, FSR, wood bleach, Muriatic Acid, Mary Kate On and Off, and most toilet bowl cleaners are basically acids which dissolve the dried organic matter and bleach the surface.
FSR, and wood bleach are both Oxalic acid, the same acid in Bar Keepers friend. They are just in different forms, FSR in some gel and wood bleach as a dry powder. O&O is a combination of hydrochloric acid and phosphoric acid and a few other things. Muriatic Acid is hydrochloric acid. Toilet bowl cleaner (at least the one I looked up) is bleach with some fragrances and other things to help clean things.
I would not bother using TB cleaner, it is just too expensive. O&O is also one to avoid because phosphoric acid breaks down in to phosphate which is a potent fertilizer, we don't need more of in our waterways as it promotes growth. I haven't tied FSR on the waterline although I have used it on other parts of the boat, because it is a gel it is easy and neat to use. Although at
$60 a gallon it is a little rich for my budget.
The $10 a gallon Muriatic Acid works well and when diluted it costs about $5 a gallon or less than a dollar a season. The disposable chip brush costs more than the MA. MA also has other uses. It very good at cleaning mortar and cement off bricks and stone and makes short work of clearing calcium deposits from clogged fittings in the sanitary system.