The official XAP does have ID_CAP_INTEROPSERVICES, incidentally - whoever removed it from the version in the OP, shame on you. That was there for a reason.
What kind of driver? How about one that hooks into the phone stack? Remember, there's no public API for talking to the phone interface, and apps don't run constantly in the background so they can't look at every incoming call and decide whether to take it or not anyhow. Instead, you need to use native code with sufficient permissions to hook into the software stack used for taking calls, and you need to do it with something that can be queried against at any time. In other words, you need a driver or service (there is a difference, but I don't know enough about CE to be sure what that difference is).
The important point is that drivers and services are typically baked into the ROM. I've tried modifying an existing driver - not even installing a new one, just replacing the DLL with an older version - to no avail. I'll try again if anything new comes up that I haven't done before, but until then, you'll need to hope for a firmware update from Samsung. If the rate at which they provide updates to their Android phones is any indication, don't hold your breath.
Q from 3 days ago: still not working on other phones????
A from 2 days ago: no :-(
DERP from today: So does this now work on HTC phones ?
...
You do the math.