If I remember correctly you can change textures and other brush settings via the config files. I have only added other texture types and localized the filenames and the info in the configs. You can create new brushes in that menu where it has all the diffrent types. Right a blank spot and choose the base type. I think I saw a list of these in a config file.
NOW what I dont like is sharing pallettes and brushes is quite a pain. Since you will have to move over a lot of stuff and do some manual work in those config files.
You can clone them by right clicking and dragging as well as relocate your brushes IE if the layout sucks you can move them around.
I havent tried making tone brushes IE the little dots you use to digitaly paint ziptone. I found Photoshop faster for this "dojin/manga" form of paint or fill. I dont know if SAI can read transparent info from BMP or what apps support that, maybe it runs off alpha? Or if SAI will see other file formats. Either way brush textures are brushtex.conf add the info there so SAI can find it.
Would be fun if you could do say picture tube painting. Like how painter does with true gradient transparancies and controls for size and random placement etc. But considering how bloated painter is in comparason it would ruin all what makes SAI great.
Hope this helps some Ill look into it more later on and post some tuts or something.
Added leather texture.