I am wondering if anyone is familiar with a HELOC product that would sweep funds from a dda above a target amount, and apply as payments to the HELOC. What special disclosures would I need? What special limitations would I need to be aware of? Would Reg E come into play?

It would be a contractual issue. I see no specific compliance issues. The transfers would be exempt from certain parts of Regulation E - § 1005.3(c)(5).

My concern specifically is if the client wants a debit card with the DDA. If he uses the debit card for a POS transaction that advances the HELOC, does this then become a credit card, subject to all their rules and regulations?

Well that is a totally different question than you first proposed as there is no mention of a debit card or advances in your first post. However, the debit card transactions would remain subject to Regulation E. See 1026.13(i).