A work-in-progress script to add MailTags keywords to messages in Mail based on sender. Uses GUI scripting and you'll have to jump through some hoops to add this to a workflow, but it may be of use. Toughest part is adding new addresses to it. Also note: the tags in the script must "Preferred Keywords" to be in your menubar.

automate-mt-tagging.applescript

# Automate MailTags Tagging
# by Jonathan Poritsky
# http://candlerblog.com
#
# Instructions for editing inline.

tell application "Mail"
  activate
	set m to (selection)
	set s to (extract address from sender of item 1 of m)
	if (s contains "@gmail.com") or (s contains "jerk@yahoo.com") then --edit email addresses in quotes. To add more addresses add another "or (s contains...)" before "then".
		tell application "System Events"
			tell menu "Add MailTags Keyword" of menu item "Add MailTags Keyword" of menu "MailTags" of menu item "MailTags" of menu "Message" of menu bar item "Message" of menu bar 1 of application process "Mail"
				click menu item "@tag" --pick your tag name here
			end tell
		end tell
	else if (s contains "otheraddress@whatever.com") then --Same as above 
		tell application "System Events"
			tell menu "Add MailTags Keyword" of menu item "Add MailTags Keyword" of menu "MailTags" of menu item "MailTags" of menu "Message" of menu bar item "Message" of menu bar 1 of application process "Mail"
				click menu item "@othertag" --Here's the next tag
			end tell
		end tell
-- Copy and paste from previous "else if" to here to add more rules for tags.
	end if
end tell