How to manually control the updates for an App?

We are looking to develop an iPhone App communicating to webservices that is hosted internally at the customer site.

We will be maintaining the code, updating the App when required and releasing to the customers, but it should be downloaded to their iPhone only when they update their backend product and the webservices.

Therefore, It is possible that the webservices at the customer site could be on a version older than what the latest app requires. For example, there could be some new methods called in the new app that are not available on the existing webservices at the customer site.

We want to control the update of an app based on the locally available webservices.

We were thinking of the following processes,

1) Leave the app on the appstore but control the update processes somehow based on the webservices version. So when a user try to update, it will not update unless the required webservices is available. (Annoying part on this is users may be prompt that there is an update available even though those are irrelavent for them.)

2) Control the update outside the AppStore. Something similar to In-house development.

3) Stop the updating process entirely from the client, and only trigger the updates from the webservices end.

i.e Inform the client that it needs to update when the webservices updated, and the client will run the update process at that point.

However, it could be tricky, if the customer's updated to the webservices version that is not the latest.

What would be the best way to approach this? And are there any documents or refference available to implement something like this.

Thank you for you help.