You can just go ahead and offer the 720p, seeing as animerg and project-gxs are usually reencodes. The 1080p will have to prove that it's worth it which will require a comparison. If you want you can try your hand at one, or instead wait until one of us gets around to doing it (since it's just a few releases it should be done quickly). The necessary guides are on the wiki and the forums.
As for requesting permission: Normally people don't do that, though it's refreshing to see someone think it's necessary.
Also in the future, if you do research on what you plan to offer like this, you don't need to ask for permission on the forums. In the worst case we'll reject the offer, but it will not reflect negatively on future offers (if I remember correctly my first attempts at offering anime were rejected
). What's more likely is that we and other users will point you in the correct direction and help you get your description sorted out in the time it takes for us to look through things.
One more thing, in the process of comparing various releases we sometimes end up with stuff we want, but that aren't on the offers list. If you ever find yourself looking for something to offer, check our Free to offer