My thought would be that the "Following"-List of each user would be anonymous and not publicly visible, and strictly be for subs you only lurk in, but don't want to publicly become a member of (so no commenting or posting there). The "Tribes"-List would stay the same, publicly visible as usual, including all the posts of the user there. Optionally, the amount of users just "Following" could be shown in a counter next to the "Tribe Members"-counter of each tribe, perhaps there could even be a "People online"-Counter in each tribe.

It's probably either something like that, or secondary Accounts will most likely become commonplace to circumvent the whole thing, which is bad for both the site and the comfort of the user.