Reg. Z requires you to base your disclosures on the legal agreement between the parties. In this case, the addendum becomes part of the legal obligation and these additional curtailments must be factored into the payment schedule, total of payments, finance charge and APR.
Because you need to estimate the dollar amounts of the future bonusses, this TIL disclosure may be too sensitive to be handled by your lending staff in the usual manner. As an additional problem, you may lack the software to adust your disclosure calculations to reflect differing curtailment amounts for the number of years required to bring your LTV into line with the bank's normal standards.
If you would like to consider outsourcing this one disclosure and having HR hold the retention copy, I'd be happy to help you produce a stand-alone TIL. This way, you will comply with Reg. Z, protect the borrowing employee's privacy, and preserve the integrity of your compensation system.
_________________________
...gone fishing.