Possible to modify {user:first_name} merge tag in notification filter?

I’m using the standard “Hi {user:first_name},” at the top of my notifications. This is fine when the user is the only notification recipient, but it presents an issue when another recipient is dynamically added via the gform_notification filter.

To elaborate, I have a form that includes an Authorized Signee Name field and an Authorized Signee Email field. Both are pre-populated with merge tags (user:display_name & user:user_email), and the values for each can be changed if the user wants to designate someone else as Authorized Signee. When the form is submitted, my gform_notification filter compares the {user:user_email} to the value of the Signee Email field, and if different, it appends the Signee Email as a second recipient.

While this works great for ensuring that both the user and the signee (if different) receive the notification, I can’t figure out how to use “Hi {user:first_name}” in the user’s notification, and something like “Hi {Authorized Signee:3}” in the signee’s notification.

Any suggestions?