Are you planning on supporting Woo Commerce Subscriptions Plugin ? There is documentation on extending the API.
If there are no current plans as part of your current development plan could you please let me know how much you would want to take this on as a privately funded project ?
If possible could you drop me an email at email@example.com . We are investigating using Woo Commerce for a client but we need Sagepay to work with Woo Commerce.