@That1Smart Guy
Have you tried
Metasequoia? That has just the feature you're asking for, you can paint straight onto the model with any brush texture. The feature is a little buried (can't remember where off the top of my head) but it is there, even in the slightly limited free edition. Might be worth a look...
@BN2
Wouldn't it just be simpler to take a UV mapped model, move a paint brush over the texture, grab a new image of texture + paint brush and apply this new image to the model to show you what you would get? Then you could click the mouse to commit the change, and the new image replace the original texture.
The way you're suggesting is great, but it fails to take into account that different faces may share UV coordinates (e.g. part of the model could be a mirror image of the rest) and the user might not realise that they were modifying more than one face.
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