I saw this suggestion elsewhere as a core change, however in the meantime I think it would make a great addon. Basically it allows an option for guests to use the quick or standard reply / post a new thread. However when they go to submit it saves the post / thread and takes them to the registration page. Ideally the page should have custom text, so you can quickly and clearly highlight to the guest that their post / thread has been saved and will automatically be submitted once registration is complete. I think this is fantastic idea for getting more guests to register, whilst not being overly intrusive.