[Baarogue]

Well, since it's become painfully obvious that this feature isn't going to make it into Omnifocus - I suspect because they're working on Omnifocus 2 - I began experimenting with the app in an effort to find a workaround that didn't involve individually deleting 100,000 completed tasks. That's not an exaggeration.

I've finally figured out how to cope with the problem of completed task bloat in OF relatively quickly and easily.

1 - Create a new project.
2 - Change each active task to the new project.
3 - Delete the old project, which also deletes all tasks still assigned to it.