Cappy’s approach is valid, by all means, and yes, it only takes a bit of time (unless you’re editing an epic and the clip you want to pre-render (or flatten or freeze—whatever your preferred terminology is) is 20 minutes in, and there are a thousand clips on the timeline before it). When we get the ability to select multiple clips on the timeline, this will reduce the time needed for such an approach, but it is still a inefficient.
Quartzn’s suggestion of setting render in/out points is a good one that wouldn’t clutter the editing interface — it could be handled when you’re setting up your render, though it could be difficult to get exactly the range you’re seeking; you’d have to note the start and end time of your pre-render, which might not be easy to verify once you’re in the render mode). I think a “work bar” or something like that might be useful on the timeline, though though it might run the risk of cluttering the interface up. (Again, depending on how multiple clip selection is handled, it might be possible to select a range on the timeline and send that off to render.)
Just some thoughts.