Customize Support Rep Notification on assignment

I want to be able to insert an alert message to the Support Rep when certain types of tickets are assigned to them.

I know I can edit the message content via Admin / Notification Rules / Support Rep Notifications / "Alert the following Support Reps by Email when a new request is created" / Customize Template.

It seems on Customize Template that I only have the ability to insert the contents from about 30 different fields. There's no support for conditional logic on the Customize Template nor does it seem like I can display user defined fields.

I have a business rule that's already firing for the tickets as they come in. At present the rule sets the

Category, Priority, and Group.

What I'd love is for the rule to be able to do something like setting the Comment field to my alert message and I could then include $Comment in the alert message when the ticket is assigned.

Does anyone have thoughts on the best way to handle this? I was hoping that could use a a business rule to fill in the value of the $Comment field with my alert message but I don't see a way to do that.