I've set my umask to
~/.profile, but this only affects command-line programs, not graphical applications like the Finder. Is there a way I can change the umask that apps running under my account use (without changing it for all users)?
I don't know of a way to do this, and I'm pretty sure you don't really want to do it. As I pointed out in a comment on your first link, I know someone who experimented with locking down the default perms (as you're trying to do), then changed their network settings and some of the network settings files wound up unreadable and the network became unusable. You may not think of System Preferences as an app that creates files, but it is and it does.