We could, for example, update the list of actions to include a "send an email" action, with customizable text.
The ability to send an email reward with customizable text would probably open up some possibilities for other forum admins, but in my case, this would only be useful if the trigger is coming from outside of NodeBB, like you said (if they're already a registered member, then any of the existing actions would presumably be sufficient to get their attention... the idea behind the email is to reach someone who isn't part of the community yet).
It sounds like I'll have to get with my developer for this one... however, the whole purpose behind this automation is so that I can send out email invites to outsiders so they can join a private group (per this other thread), so we'd have to get that squared away first before this automation step makes any sense to work on. I'll email you about this shortly. 