I've been looking at how to get Broadcast Message Expiration and Replied to Guest to install without resorting to a combo mod. 

My general observations - the easy way would be to include Broadcast Message Expiration in 2.7.00.

The right way is to refactor the entire Personal Messages section and separate Broadcast Messages from Guest Messages. Currently they are utterly conjoined even though they are actually 2 different things that have to be treated differently - guest messages can only be replied to via email and Broadcast messages can't be replied to at all. (Seriously - there are multple if/else statements scattered throughout multiple files just to separate out which actions are allowed with which type of 'BM'.)

So this may take awhile. (JonB and I did have this discussion a long while back and decided to 'temporarily' patch the old bugs in Guest Messages for 2.6.1x. But now it's pretty obvious it needs to be done for 2.7.00 so it will be right when we start working on mySQL.)