(I have solved my problem, so have edited it to look more like a blog post - the final working solution has been placed as an \"answer\" below.)
I want
I have managed to solve the problem so have edited the question, and placed the final procedure, with notes, down here:
1) Change the SDK project to be a MIDLET application as per this link (Richard Schilling’s Professional Weblog)
2) Build the SDK project in Eclipse. Package & Sign the project as normal.
3) Copy that resulting JAR file into your project, and update the build path to use that JAR.
4) Also in the build path settings, go to the "Order & Export" tab & check the box to export this JAR file.
WARNING: at this point a lot of users seem to encounter the bug where Eclipse tells you "Project {0} has to have a jad file file if it exports a mildlet jar". This error happens if you do not set the SDK project's type to MIDLET.
If you get this error, please check the project's build type.
If you have set it to MIDLET correctly, try editing the manifest inside the JAR file, to remove the line MicroEdition-Profile- MIDP-2.0
.
Full details are given in this link: "Project {0} has to have a jad file file if it exports a mildlet jar" error appears in BlackBerry Java Plug-in for Eclipse
NOTE: regarding the RIMAPPSA2 signature, I found some info about getting rid of that requirement. The steps seem strange, but they worked for me insofar as the signing stage went. Unfortunately for me the app didn't run on the phone (still requiring some signatures). But these steps might help someone else & can be found at Frustrations with Blackberry Developer plugins for Eclipse
NOTE 2: remember to refresh your project after reimporting a new build of the JAR file, if you have to go back to step 1 (otherwise Eclipse will keep using the old one you have just overwritten).
So to summarise:
If you get questions about RIMAPPSA2 signatures, see step 1.
Easy when you know how - but this took me many days to get right?!?