More Googling has uncovered 2 new ways to invoke Java/Scala code with MetaTrader. Both are commercial products but licenses are reasonably priced.
The first takes an approach similiar to the one demonstrated in this blog posting. The name of the product is JavaDllBuilder. It compiles Java code into DLLs but uses a JNI translating layer to make the code available.
The second is more interesting. It is called DDE for Java, a DDE client for java (naturally!) Using the approach of a Java program accessing MetaTrader through DDE I think I can take advantage all of the heavy lifting done by MetaTrader (indicator calculation, actual communication with the broker) and do all of the fun work in Scala!
Edit: crap -- DDE is being dropped from MetaTrader 5. Scratch that option!