If you happen to have AWS S3 that works like a charm also. Well. Relatively speaking :-)
Create a bucket for your ad hocs in AWS, add an index file (it can just be a blank index.html file) then using a client that can connect to S3 like CyberDuck or Coda (I used Coda - where you'd select Add Site to get a connection window) then set the connections like the attached:
Then build your enterprise ad hoc in XCode and make sure you use https://s3.amazonaws.com/your-bucket-name/your-ad-hoc-folder/your-app.ipa as the Application URL, and upload it to your new S3 bucket directory.
Your itms link should match, i.e. itms-services://?action=download-manifest&url=https://s3.amazonaws.com/your-bucket-name/your-ad-hoc-folder/your-app.plist
And voilá.
This is only for generic AWS URLs - I haven't tried with custom URLs on AWS so you might have to do a few things differently.
I was determined to try to make James Webster's solution above work, but I couldn't get it to work with Plesk.