Excellent question! As far as I'm aware, standard GTD methods don't actually handle this type of thing all that well, so I look forward to others' answers.
I would probably do one or more of the following:
I might approach my NA list thinking, "Let's see -- I really want to get that creative project done; let me just check whether there's anything more urgent than that that I need to worry about in the next couple of days."
- Put it on NA lists with an asterisk and a mention of the deadline, e.g. "for Friday"
- Make one or more appointments with myself: e.g. two 4-hour time slots in case the first one doesn't work out, and move them rather than cancelling them if other things come up; and schedule time to eat just before etc. The idea of appointments with oneself is mentioned in the GTD books although it seems to me to run counter to the other GTD principles.
- Keep remembering all week that this is important, with a feeling of determination and creative energy.
Inability is an abstract thing involving comparison with alternate universes; it cannot be experienced.