What about installing only pieces of NetWeaver?

Greenbaum: If you want to have NetWeaver functionality -- such as MDM -- or run an xApp, you can do so by running a separate instance of NetWeaver independent of the rest of your R/3 implementation.

You can still have interoperability with the rest of R/3 via the XI layer or the portal, though but it won't be as clean as if you upgraded the entire environment to MySAP Business Suite and NetWeaver.