I'm thinking of doing a garbage collection at boot, to avoid locking up the entire system for a garbage collection while saving a file.  Thoughts?

I would estimate it at 10-20 seconds.

Ooo, it might be a good idea to freeze file streams when garbage collections are in progress, instead of only allowing one file at a time.  This is something I've been struggling with: how to edit multiple files at once?  I know I will probably use filestreams to make it easier on the user to cross page boundaries and such.  But I'm still not sure how to handle multiple pages.

I think that a solution to the multiple files problem is making programs define the size beforehand.  As for programs handling flash directly, I would prefer they use filestreams in the OS to handle it, because the OS will handle the page boundaries.  A filestream is a means to reading and writing files or memory, or in KOSes case, only files.

It will probably be scrapped, seeing as most files will probably end up being too big.  I don't want to edit files that take up most of available RAM.  In KFS, three bytes are dedicated to filesize, and I want to make sure I support that.
As for editing multiple files, if two files are growing in flash alongside each other, there will be significant problems unless I garbage collect every time a byte is written.

Delete+move would mean that the garbage would pile up very quickly.

Yeah, I'm not sure about that.  I'll think it over for a bit.  Imagine if two programs were writing large files, multi-page, perhaps.  It might garbage collect 10 times over the course of saving them.