I've just had another go on my own, and by downloading the HDK from the Apache Harmony download site and adding the "x-net.jar" from that to the build path it appeared to pick it up.
However, it's now complaining that "The method getDefaultTrustManager() is undefined for the type SSLParameters".
The method definitely exists (at least in the Android platform) because the Email app calls it, but it Eclipse doesn't seem to want to see it.
I have the HDK for JVM 6.0.
What am I doing wrong?
Edit: Added the jar name