Yep, you can do it. You'll be able to edit until sign-ups close.
If you're really worried about getting trapped offering a blank request, you can also create a TBA DW entry, and link to that in your sign-up. That way, even if you somehow miss the closing of sign-ups, you'll have something you can edit. (ETA: And now I see above that you've already thought of this.)
I was going to point you to the Automagic App, in case you didn't know about it, for easier tracking of requests, but it looks like the mods haven't fully set up the exchange on on AO3--there are no dates in the profile--so I'm not sure the app will be able to scrape it.
no subject
If you're really worried about getting trapped offering a blank request, you can also create a TBA DW entry, and link to that in your sign-up. That way, even if you somehow miss the closing of sign-ups, you'll have something you can edit. (ETA: And now I see above that you've already thought of this.)
I was going to point you to the Automagic App, in case you didn't know about it, for easier tracking of requests, but it looks like the mods haven't fully set up the exchange on on AO3--there are no dates in the profile--so I'm not sure the app will be able to scrape it.