Yes, similar to the site wide  button. The only dissimilarity being the its association with any given space would have to be obvious and not be mistaken for the site wide button

To be clear, every single parent/child space should have an 'add content' button.  Most end users will expect it to work this way.  I'd suggest putting it wh.ere the 'Leave Space button is now, and push the leave button all the way to the right.  The current method is fine for adding top level content, but when that content belong to a unique space, group, event, etc., it needs to be treated differently. It doesn't mean you have to change everything about the existing method, just add a new method. Multiple ways to accomplish the same thing is rarely a bad thing.  I do believe this new method will have a big positive impact on end user understanding and usability of una based sites.

If it takes custom paid work to get it done in a hurry, I'll be watching for a price quote.  I have a complex application where I will have 50 or more top level parent spaces, and eventually thousands child spaces in the hierarchy.  The idea is unworkable without being able to add content by a direct method.

I think "Add content..." block behaves almost like this, what is your variant compared to existing "Add content..." block ? or how your variant should work in more descriptive way, it would be great if you can make some sketches ?

Also we plan to redesign Privacy field to make it like a popup, so you will be able to search for groups/spaces/etc where you can post.