Assign to Technician or Group based on email reply content

I am getting ready to implement SDPlus (on-premise) to replace another help desk system and I am having trouble trying to figure out if one of our desires for the system is possible.

Here's the scenario:

We have three support groups in IT: let's call them A, B, and C

All new tickets come in to Group A.
An email is sent to all technicians in Group A when a new ticket is received into the group.
Group A analyzes each ticket and determines if it stays in Group A, or must be assigned to Groups B or C (or to an individual technician).

What I want is to be able to take an email from (2), reply to it, and include some key text, like "assign-it-groupb", then have SDPlus change the assignment to Group B.

Many times our technicians in Group A are not near an SDPlus console, but are able to reply to an email on their mobile device.

I have tried creating a Business Rule to look at the Subject and/or Description, but it appears that doesn't work on an email reply to an existing ticket? Maybe because it's in a Conversation and not part of the regular body of the ticket?

Any help would be greatly appreciated!