Well this was easier than expected, so i am happy to say that dynamic "ReplyTo" support will be available in release 1.3 (just waiting for a new ID file). Testing of this function in Notes r8.5 went well, and everything I've read suggests it will work in Notes r7 as well. It only gets tricky when trying to force Notes to use a different "Sender" or "Principal" address (and I won't be going there anytime soon!).
Note also that in the existing code, if you can set the ReplyTo on the original Mail Merge 'draft' template email (by using the 'Delivery Options...' dialog, 'Advanced' tab), that reply address will be copied and used for all generated emails as well. So this dynamic "ReplyTo" function in v1.3 is only required if you want that address to be different for each generated email.
That said, instead of depending on any Reply-To functions, I would still recommend you use the Mail Merge functions from a generic Mail-In database. From a cursory glance, it would seem that this Mail Merge project would be fully compatible with the DominoTeamMailbox mail template, which includes a superior spam protection and a lot of other cool features. I might have to make a suggestion that these be rolled together!
Hope that helps and thanks again for the suggestion.
Please remember to rate and review :D