Is it possible to set up a reverse DNS record on AWS for address from a different registrar?

From a technical perspective it doesn't matter where the forward DNS is hosted.

The reverse DNS is typically hosted by the provider of your IP-address, so in your case by Amazon. As long as the forward DNS matches your actual ip-address and the PTR-record you're requesting, Amazon allows you to set-up a reverse DNS record by contacting them.

Q: Can I configure the reverse DNS record for my Elastic IP address?

Yes, you can configure the reverse DNS record of your Elastic IP address by filling out this form. Note that a corresponding forward DNS record pointing to that Elastic IP address must exist before we can create the reverse DNS record.

Yes, it doesn't matter.

Simply setup your reverse DNS zone on your hosting or on the domain itself using a zone and then a PTR record and then contact them here:

https://aws.amazon.com/forms/ec2-email-limit-rdns-request

You'll fill in the IP and the host and they will then approve. Ensure it is setup correctly, else they will not approve the reverse DNS.