I've kinda found a way to use 999 soul when you only have one in your inventory. You have to by 999 of an item (regular wood arrow for less than 3000 soul) and then do the "glitch" to be able to use 99 consumable soul unstead of one but it will use 999 soul due to the 999 items bought. I don't know if any one esle found it yet and the comunity decided that it was not worth the time or if it was to over-powered or anything else, i just wanted to share and didn't manage to record it myself. I am just a viewer, not a speedrunner since i don't have the determination to speedrun games. :p

you could check a run to see if it's used.

It's commonly used in runs, and was used the moment it was discovered, as far as any% is concerned we use the arrows from Andres shop to store the max quantity, and then use that saved value to duplicate the Soul of a Proud Knight we grab in the Valley of The Drakes, can confirm its known in the souls community :P

Edit: Good looking out though, if you ever find anything interesting that the souls community doesn't already know, definitely let us know!