How to make available iOS app to different client's MDM
When it comes to AirWatch, I as MDM admin have the option to upload a file, containing the application. So you would have to deliver your IPA file to your client. Now, it's just me guessing, but I would believe every MDM that allows to publish internal apps has this option. 

Another option, and I guess it would be the preferred one, is managing your app as B2B app in the VPP program. Here is a write-up I found of how the process works.

http://www.techrepublic.com/blog/apple-in-the-enterprise/how-to-purchase-and-deploy-custom-b2b-apps-through-apples-vpp/