Reg Z states:
15(f) Exempt Transactions
1. Residential mortgage transaction. Although residential mortgage transactions would seldom be made on bona fide open-end credit plans (under which repeated transactions must be reasonably contemplated), an advance on an open-end plan could be for a downpayment for the purchase of a dwelling that would then secure the remainder of the line. In such a case, only the particular advance for the downpayment would be exempt from the rescission right.
You could advance the funds for purchase immediately as they wouldn’t be subject to RoR, but all other advances would subject to RofR, so one option is to get a right of rescission for all future advances at closing and wait until the rescission period has expired before advancing any additional funds.