I have just about managed it using Ctrlr, which I had looked at in the past and found too confusing to comprehend. Now I can control the synth using a VST interface within Reaper. There's just one last niggle of working out how to make the automation work, which I can do with audio tracks and with VST instruments, but this Ctrlr one might take some working out.
But generally, a success, and for free.
After that I need to work out how to map the VST parameters to my BCR2000 (or Remote SL), and I'm good to go.
Edit: that sounds confusing even to me.
Basically, it works by adding Ctrlr as a VST instrument to the midi track in Reaper, then opening a PG-300 emulator within that VST, and the parameters on it now control the parameters of the a-Juno.
Automation yet to be worked out (all the parameters in the Track/Envelope automation window just have numbers rather than names, and don't seem to be responding to anything) (but I'll work it out).