You can CNAME a subdomain you control at Amazon S3 (to a bucket with the name of the subdomain), like so:
http://s3.ceejayoz.com/ (goes to my 's3.ceejayoz.com' bucket)
Uploading your own clientaccesspolicy.xml file to the root of that bucket (and setting the permissions to be globally viewable) should do the trick, if I'm understanding your question correctly, as it will be accessible at http://s3.ceejayoz.com/clientaccesspolicy.xml.
More information in the S3 docs: http://docs.amazonwebservices.com/AmazonS3/2006-03-01/index.html?VirtualHosting.html
edit: From looking at that, you could also use the "Example Virtual Hosted Style Method" without a CNAME: http://bucketname.s3.amazonaws.com/clientaccesspolicy.xml