Are participations purchased excempt from ECOA; specifically Reg B timing requirements?

Not sure what your question is regarding timing. Participating in a loan that is in violation of Regulation B of course could present some risks. If you are talking about a bank asking you to participate in a loan and you fail to respond within 30 days, that is not an issue. You are not granting credit to the customer, you are only agreeing or not agreeing to participate in the loan made by the other bank. Any adverse action would be up to them.