FYI, there is also the MandrillX class, though you'd still need a custom FormIt postHook to run it. Notify has been using it for some time and it's very reliable. http://modx.com/extras/package/mandrillx
You might look at the code of Notify. I'm pretty sure it uses a Mandrill template.
Sorry, I was reading that as using a MODX template to send through Mandrill.
IIRC, Notify sends the template once to Mandrill, then sends the data to use with it. I like doing it that way because I don't have to log in to Mandrill to alter the template and it still avoids sending the whole text for each outgoing message.