I personally have no experience with Live, but I do know a fair bit about designing successful standards and protocols... Make it as open and well documented as possible. The more accessible the information on how to implement the protocol is, the more likely it is to be adopted. I've seen a lot of great technology fall due to the fact that it was poorly documented, or alternatively the documentation wasn't made freely available. Making a custom protocol is a double edged sword. On one hand it gives the greatest flexibility, but then again adopting an existing open protocol (Open Sound Control, for instance) could be a good option as well. Not that I claim to be able to judge the suitability of OSC for an application like this. Also, don't make people buy your product to be able to interface with it. I don't know if there is already a suitable demo available, but it makes sense to have a full-featured version of the product with a full protocol implementation freely available for download. (Saving disabled or something like that.) This way someone who wants to try interfacing his DIY controller with your product can give it a shot, and you might end up gaining a customer.