This would be nice, indeed.
In Moodle 2.5, I see that there is a 'moodle/user:ignoreuserquota' capability. That could be used to ignore the quota completely for someone with a given role.
It seems to me that the following setup would be better, though:
* add a maximum user quota setting (e.g. $CFG->maxuserquota)
* allow admin users to configure a custom per-user quota (up to $CFG->maxuserquota) for other users (by editing the user's profile)
Then by default, users could use up to $CFG->userquota space, but some users could be allowed to use up to $CFG->maxuserquota space.
What do others think? Would something like that be useful for your Moodle?