siiigh, ya not that i know of. and only being able to have up to 64 pixels per brush shape (you know when u can make ur own with that crosshair lookin thing) is kinda difficult too.
What i would do, is id just make make folders seperately to categorize all ur textures, so u can just swap them in and out when u feel like it.
(also theres supposed to be a new updated 'Sai2' coming so that might change some things for the better)