some ideas
This was some years ago, before the email spam flood-gates broke, but here's how I had a sub-domain set up...
A buddy of mine who worked for Verio, set me up with one of their domains: ni.net
He rigged me up as: The.Knights.Who.Say.Ni.Net
Any mail name @ that domain was forwarded to me.
The likelyhood that anyone would stumble across the subdomain and spam-flood it, was minuscule.
...so long as I didn't publicize it...
Of course, if you post the entire subdomain on your web site, then you're screwed, of course.
Is there any chance you could set up a subdomain to act as a filter for your needs?
eg:
catchall@unique.mydomain.org
...or, without the catchall...
The other thing I like to do, is let's say you send in a rebate to Reebok...
Make a mail forwarding account as:
ReebokRebate@mydomain.org
At least, that way, you know who leaked your address, and you deactivate that mailbox.