There's already a Subscription Manager role, whereby you can designate anyone in the system as Subscription Manager to have access over subscription options (this may only be in OJS 2.3; I don't remember off the top of my head). You can also email subscribers individually from your individual and institutional subscription lists (again, this is only in 2.3+). However, you can't currently mass-email subscribers. I've added this as a feature request here: http://pkp.sfu.ca/bugzilla/show_bug.cgi?id=5190;
please CC yourself for future updates.
(You may also want to take a look at OJS 2.3's subscription options, if you haven't already. We'll also be releasing 2.3.2 in the next few weeks, which will be considered a stable release and may be worth looking at as an upgrade option.)