The architectural problem you are facing is that SAP is not a data source. SAP is the level of business logic. Trying to make it act as a simple data source is likely to cause problems in the future.
So my advice is to use simple WCF services. Or, if you are using an old version of sap, look forward to a new new .net connector . Make a presentation layer in .net or silverlight. And keep all the logic in the juice.
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