To make five patches to submit, I created five branches based on
master, then rebased each one leaving only one of the feature commits
for each, thus ending up with five patches starting in 0001. Is this a
reasonable way to work, or is there a better solution?

Yep, that works fine. The number doesn't really matter to the receiver.

Also, I submitted all five patches at once, with git send-email, but
only four made it through. The 5th patch ended up bouncing back to me
with the message CORRUPTED MESSAGE. This has happened to me before,
but I found it very odd that it would work fine for 4 out of 5
patches, all submitted with one command.

That's odd. The only time I have seen that is when the patch contains *very* long lines, such that git send-email chokes on it. In that case, the solution is to communicate the patch via alternate means like posting to a webpage, gitweb, FTP, etc. It may be helpful to update your version of git if not current, also.

Glad to see your contributions hitting the patches list!
--Joe