Hi,
wouldn't it be an option to just consider an extra call per thread returned by search? I believe this would be acceptable from a performance perspective (assuming the search result page is paged)
For example
http://lithosphere.lithium.com/restapi/vc/threads/id/129799?page_size=1
(we use page_size=1 as we are not really interested in the details of all the replies) - will tell you how many replies your thread had and the details about the solution (if available). If we search for a topic which has no solution (this one, at the time of writing :-) )
http://lithosphere.lithium.com/restapi/vc/threads/id/62298?page_size=1
We can see that there is nothing in the solutions element.
Thanks,