It's not officially possible.
You could try Enterprise distribution as others have mention, though it's not designed for distribution outside your company. I'm not saying it will happen, but if Apple find you're doing it they might close your developer account. I've never heard of that happening; only you can decide whether it's an acceptible risk.
Apple would say that you should put the app in the App Store. Unless Apple would reject your app I don't see why this wouldn't work. You don't have to make it easy to find and you don't have to make it cheap.
You can limit it to your customers by requiring a log in. Many, many apps do this, from Salesforce to Skype to Twitter.
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