I was just trying to figure out this exact issue. Larger projects with subrepos really amplify this issue as well.
It would actually be quite nice if the deploy key interface worked like the user interface letting me create groups and each group has keys in it etc. Alternately letting a group admin create deployment "users" that can be added to the regular user groups would also work fine.
Update: I believe this is already working . I just upgraded my plan from the free 5 person plan to 10 and now on my manage team page i see an option to set an SSH Key. I remove the one i set on all my individual repos , moved it to here and everything seems to work fine!
Strange that this ticket would stay open for 3 years ...