Function libraries for email templates?

Is there a way to maintain libraries of our own functions for use in the email templates?

“Don’t Repeat Yourself” (DRY) is an important programming principle, but our templates are built with a lot of cut-and-paste snippets.

For example, we frequently use:

{{ person|lookup:"VehicleModel"|default:'ride' }}

...to say things like “new items for your Toyota Tundra”. This is sprinkled through a great many templates.

The right way to do this is with a function, something like:

def getVehicle(person)

  # return person|lookup:"VehicleModel"|default:'ride' 
  person.fetch("VehicleModel", "ride")

end

...and then just

{{ getVehicle(person) }}

in the template itself.  It’s short, simple, reusable.

is this syntax, or something like it, possible? Ruby, Python, Javascript - I don’t care which, any reasonable language will do. And can functions be placed into a file to be shared and reused across all our templates?