Some ideas (need to work on the implementation)
Could the enrolment key get pulled from the database and put in the email via php? It is stored in the mdl_enrol table under the field password and tied to the course via the courseid in that table.
In theory, you could set the key and enrol somebody in the course using the key (that will put it in the database). Then pull the key from that place in the database and stick it in the welcom letter.
Wonder if there is a Moodle variable for the key OR if a variable could be created using the above logic.