BTW, this feature was implemented. 
Well, not exactly, since it’s extended to trust levels and not groups, but it’s good enough, since you can restrict the Shared Drafts category to the groups you want.
There are two settings you can use:
| SiteSetting | Description |
|---|---|
shared_drafts_category |
A specific category dedicated to shared drafts. It must exist before using this setting |
shared_drafts_min_trust_level |
Who can see and edit shared drafts (I guess it should match the above category’s security settings) |