Essentially you need to do two things:
First, you need to get the custom USB device working with linux. Eventually, the linux underlying the android stack on the tablet. But first, you probably want to get it working with a desktop linux where it is easier to experiment interactively. Probably this will involve loading or even modifying a kernel driver, and possibly setting up rules to have it automatically loaded. Once you have that going, document what you did and do the equivalent for the linux of the android tablet. This almost certainly requires root. In the android context, the method of autoloading the driver on connection may be different, but the driver itself would be essentially the same, only compiled against the appropriate kernel.
Second, you need to make the device available to application programs. If you only desire to make it available to custom programs, you may be able to do this via direct interaction with a device file, or creating something in /proc or /sys. Controversially, those files would have to either be world accessible, or you would have to patch another special group ID/android permission pair into the stack and let that group own the file to make it available to applications which have that android permission. A more sophisticated approach would be to write a low level service which manages the device, and exposes it to android applications via the usual binder-based android service communication mechanism.
