Normally when I need to create a PTR record for a customer's Public IP, I create the record with the ISP. As typically most customers just lease their static IP from an ISP.
Where do you create this record when your customer own their own IP block? GoDaddy.com hosts their external DNS zone. But there is no option to create a reverse zone, or, PTR record with them. I confirmed with GoDaddy support. The ISP says they don't own the IP so they can't help either.