I'm testing the bulk merge feature as a possible solution for sending out notifications to our customers.  I'm running into a problem trying to include a variable from the customer record in the mail merge I'm sending to contacts. 

In reading the help - this appears to be supported:
"In general, the kinds of tags you apply to a template must match the record you are sending your message to. However, if you are creating a template for communicating with contacts, you can refer to fields on the parent record each contact is associated with. "

I think the problem is that the field I'm trying to get at exists on both the CONTACT and CUSTOMER records.  It's an "external ID" field that we use to tie each entity to its counterpart in another CRM we maintain.