If a particular loan is not subject to the Truth in Lending Act (for instance, no finance charges are associated with the loan and it is scheduled to be repaid in four or less installments), if the loan goes into default, does charging a default rate of interest (assume this rate is reasonable and the default was unanticipated) subject the loan to TILA disclosures?

Did you just ask if a loan not subject to Reg Z is subject to Reg Z?

IMO yes. You can't charge the default interest unless you contracted for it, therefore the loan is subject to interest charges. Just my 2 cents.