Can I configure low disk space warning to a lesser threshold?

The notification is part of the gnome-settings-daemon, you can change its behaviour in gconf-editor. To do so:

1. Press Alt + F2 and enter gconf-editor
2. Go to apps > gnome_settings_daemon > plugins > housekeeping
3. Change free_size_gb_no_notify to 0.05 (for ~50MB) or whatever you'd prefer.

There is also a bunch of other settings in there you may find helpful.