In our communities, which have extensive private categories, we are finding the limitations on oneboxing a bit of a problem.
It means that we can’t easily do nice internal linking in any of these categories. As these contain around 50% of our traffic, this is quite a limitation.
It would be great if we could onebox links from the same private category (actuay we can now it seems), or also from a parent category(ies) (which by design must share the same permissions).
Returning to this as I am shortly going to ‘open up’ a couple of forums so that they have at least one public category.
Alternatively, I wonder if we could have a warning pop up (maybe with a confirmation modal) when someone attempts to onebox a secure category rather than an all out ban.
Another alternative would be to have a code of some sort before or around the link to force Oneboxing despite permissions.
This can be worked around by moving the topic being referenced in the onebox to the same category, rebaking the post, and then moving it back again. Bit of a pfaff though.
The other (and probably better) way to work around this is to quote the post of interest of course.
Would it be possible to write a plugin to override the default behaviour on this?
If so, anyone interested in some dev work?
Has anyone figured out workarounds for this? We also use many private categories.
I’m not sure that there are any apart from mine above 
(have tweaked these)
Bumping this because I bumped into it today. A staff member recommended a certain topic not be viewable by search engines, so I made a TL0+ category, and oneboxes to that category fail, even though anything/anyone not-anonymous can see it.