ODBC proxy/rewriter

I have several ODBC (and JDBC) requests, all pointing to the same server. They are all queries coming from the same database user. However, depending on who the caller is, requests should be "rewritten" to point to a different schema (for Oracle) or database (for SQLServer). In short, the identity should be proxied, because I cannot manually change it client-side.

Can this be done transparently? Does anybody know of a tool that does it, even commercial ones?

I thought about using a simple network proxy and redirecting to different RDBMS instances, but that is not a scalable solution for us; we must be able to separate these requests but still keep them on the same instance.

I can't find anything on Google, I wonder if the LazyWeb can help...?

UPDATE: I realized I need something similar to Context Switching in SQLServer 2005/2008, to wrap all my requests with a "EXECUTE AS" request. I wonder what the equivalent is for Oracle.