First off, great work on the addon! I'm quite simply thrilled that something like this is available for Blender. I hope you're able to continue the work and I'm looking forward to future updates.
So, I plan to use a large number of expressions, face units, visemes, driven facial poses, etc. and I'm probably going to reach the driver expression length limit of 256 characters. From what I understand, using handlers will avoid this problem. However, when I use handlers it causes Blender to use a lot of CPU just sitting idle. I'm assuming this is due to the bones constantly getting updated when the various handlers are called. In any case, I'm concerned that I won't have much CPU left over to do actual animation, etc.
Instead of using handlers, have you tried or considered using a custom driver function? You could pass to the function the context needed for it to return the value of the property. I'm thinking something like:
evalProperty(rig, bone, transform, axis)
If you check the "Use Self" box in the driver, you might not even have to pass the entire context to the function, although I haven't tested this.
This would also avoid the expression length limit, and might be faster than using handlers.