One strategy might be
Kourosh Dini's "delete when complete" trick from the Omnifocus Setup event.
Essentially you'd create a sequential project with (at least) two tasks.
Make the first task something like "Iterate design project for 40 minutes (Delete when complete)". Set it to repeat, start again after 30 minutes (or whatever) after completion.
Make the second and subsequent tasks whatever you need to do after you've finished (e.g. email draft to Jim).
So, in context mode (assuming you've set both of these to have contexts), the first one is available, the second one is not.
So you work on your project for a while, then check it off. Now, the item "Iterate design project" gets duplicated, but isn't available to work on again yet. It won't become available until after the interval you set in the repeat "start again after", so set that appropriately (e.g. you only want to see it once per workday, so set it to start again in 12 hours; that way if you work on it at 11 a.m., you won't see it again that day at work but it'll be available the next morning).
When you finish, you delete that task. Now the next task you set up (email draft to jim) becomes available.
So, you get the sense of accomplishment by checking something off. You can set it up to, after working on it, disappear from your list for a while so you can focus on other things, while being assured it'll resurface as often as you need it to. And you can plan the rest of the project so you know what to do when you're done with the open-ended part.