By trying setting it up both ways, I have established that I can send PMs from a Q2A 1.4 beta 2 which is standalone, but not when it is integrated with Wordpress.
When integrated with Wordpress, the link for sending PMs does not appear at all, and neither does the option for admins to choose whether sending PMs is allowed.
Is this an intended feature, and if so why?
Apart from the slight complication that the user data now resides in a Wordpress table rather than a QA one, I can't see any reason to disallow it.