Hi,
We've done some research on this and have found there is no standard feature which would enable us to set this option. I'm presuming the feature doesn't exist as a flood-check measure. Currently you are able to copy up to five members on a single private message, this also limited to protect you from spam.
We've vamped up our new member registration moderation in an effort to keep spammers from being able to post and send private messages, so I think we could consider raising that limit if it would be helpful. We've also found a third-party mod that includes the addition of the "reply all" feature. I'm a bit hesistant to try it yet, as the creators are still working out some bugs. The most worriesome is one user reported after installing the hack, all private messages were deleted from his boards.
I'll keep my eye on further developments. Meanwhile, it would be helpful to hear how many others find the current method of cc'ing rather than "reply all" is bothersome.
Annette