So an external user must have a "publicly" available calendar the local sogo instance can fetch and provide the F/B for the external one.
dont think thats a good idea, in fact everyone can be able to see it even if he does not have sogo but the just the calendar URL.

Dont think thats good and no other software does this.

i havent tried it but what happens when you make a calendar public and add it as an external calendar to your 2nd sogo. then you should be able to see F/B Information, right!?!?

1) I don't want the whole calendar, I just want free/busy information.
This excludes any titles and descriptions.
So you just have to transmit the necessary information.

2) If you have several hundreds of contacts, you don't want to add an calendar for each one to your SOGo calendar list.
And not only because this would be a lot of work to set up.
But you would also generate a lot of synchronisation traffic, which is not needed most of the time.

This free/busy information should only be looked up, when I invite this person to an event.

3) You are right, in order for this to work, the owner of that external calendar has to give you access to his free/busy information.
Perhaps there should be some extra fields for an user name and password for this access. With that the owner would not give this info to the whole world.

i think its not a very hard to get working problem using 2 sogo instances.
and there needs to be some kind of encryption. like Public-Key Auth. where both sides encrypt their F/B Information just for the "second" sogo instance, because this information should be "private" (not public) for everyone.

thats just like: oh iam on vacation....(you might want to robber things out of my house?)

do you know what i mean?

in general i think this is a somewhat great improvement, but not without some kind of encryption between "allowed" sogo instances or persons in them! but you could reuse the already implemented ACLs which are used for calendars but the "whitelisted" sogos would sync them so you can continue adding persons from the second instance to your allowed calendar people - just like a sogo-cluster ;)