When ds has playdates I have had to structure them more than I would have had to for my dd when she was that age.
Eg I might set up an activity like play doh which they can do alongside each other, or decorating biscuits, or playing a board game and so on. As ds's confidence grew I would just be around in the background, cooking or mumsnetting or similar and keeping an ear in their play watching for flashpoints. We now have a few friends who can visit without me being a nervous wreck and without rows or awkwardness. He just needed to rebuild his confidence and develop his social skills a bit (Ds's problem also began with a rather nasty bullying case).
If your dd is a reasonable writer I think a penfriend is a brilliant idea. Writing doesn't require an instant response so they can take their time to consider how to express themselves (much like Mumsnet 
). Maybe you could put a request out on Mumsnet for someone she might identify with and enjoy writing to.
If she loves drawing then my ds might like to exchange drawings with her. He is a keen "drawer" and would love to receive envelopes of drawings and to send them too. Don't worry if you don't think the idea would work for her but if it would then pm me your address and I will pm you mine.
We tried Martial Arts with ds but the particular one available to us locally (we are very rural so the options are limited) required a lot of shouting which completely horrified ds. I know it has worked well for other children though.
Ds also went to a Drama class and that was fantastic for him. He loved it, much to my amazement, and counted down the days to it. He sometimes struggled to perform but it still did him the world of good. Unfortunately his class was merged with a much older and longer class and we didn't feel he was ready for the length of session (2 1/2 hours) or the age span of the class. His drama teacher felt the same. He was very 
about it but he will be able to rejoin the class when he is older.
The psychologist has taught us some breathing techniques which we have taught to ds. The two most successful ones were to imagine he was blowing bubbles or blowing up a balloon and the other was to imagine he was butter melting on hot toast. These have helped to calm him at bedtimes and times when he has become over anxious or is threatening a panic attack.