Dallas,
I suspect this question may have been better asked simply: "How do you specify LDAP controls using the Write LDAP assertion?"
The assertion uses LDIF, so the question can be further refined to "how do you add controls to an LDIF?" To which I would refer you to the example at the bottom of https://en.wikipedia.org/wiki/LDAP_Data_Interchange_Format.
version: 1
dn: o=testing,dc=example,dc=com
control: 1.3.6.1.1.13.1 false cn
changetype: add
objectClass: top
objectClass: organisation
o: testing
Notice the "control:" line where they specify an OID, its criticality, and the data. I recommend reading section 4.1.12 of the RFC (http://www.faqs.org/rfcs/rfc2251.html) to understand the criticality values.
Please post back if you have success!