IIRC those are handled on the vRA Appliance as Velocity templates with a template pack for each scenario... This was in 7.0 (or maybe even 6.x.....?). Vmware did at one point have a cusomtization pack for those templates but it involved editing Velocity (Java based markdown style text) and having the right pack in the right place.
At the moment if I need a custom email I do it as part of an XaaS or using an EBS workflow.
There is no way to do this, you'll need to use workflows. We had the same requirement but its been in our backlog forever and we never ended up implementing it.