I coded a read quick and easy mod which sends multiple PMs to a list of users seperated by ,'s. Considering a user would have to copy-and-paste the message multiple times to send it anyways, I don't see how this is any more of a resource issue than allowing them to send it once and generating multiple messages. Your suggestion of additional clutter assumes that users would forgo sending a message to multiple users simply because its a hassle to do so individually.
To make it work, I added a small loop around the sending code and change the error messages to append any errors to a string (along with success messages). If there any any errors, then the user is told which messages were sent and which failed (any why).
I'll probably code up the ability to see who else got the message and allow for a group reply, but thats a little more involved as we would have to do more work.
