Thanks for the feedback, Nilestreet. I was under the impression that PLupload, itself, already includes a Flash fallback, even in the WordPress Media Manager implementation.
What I can't yet find is where the chunk_size setting is controlled in WordPress's core implementation (if at all). In stock PLupload that setting turns off chunking when set to 0...
The only other solutions I can find involve either a separate application (e.g. for ftp) or a Java file manager / ftp client (and these days most people are busy disabling Java).
Between the limitations of browser and http uploading (I think 2 GB, including AJAX) and host php limits, it seems we're screwed. (Edit: hmm, maybe 2GB is just a default Apache limit.)
I'm trying to figure out how Youtube does it -- do Youtube users always need a plugin/app or not?
It seems they use "chunking" in at least some of their current setup.
My own immediate need is just to upload a ~250 MB file through WP admin, but I'm stubbornly trying to get it done without ftp, as a proving ground for regular user uploads.
I guess I'd understand if this would all be solvable by moving from shared hosting to VPS or dedicated (even though my shared host doesn't have ftp upload limits), but I'm not entirely sure it would all "just work" even then.