Just for reference, after months of frustration and virtual defeat:
On bootup, DP (I don't know about other sequencers) does some sort of diagnostic routine to ensure compatibility of any new plugins it finds before mounting them .
In my case it had initially found some sort of problem, and thereafter just ignored the plugin. No amount of reinstalling the program/plugin package could over-ride this initial assessment and rejection of BRASS.
However... Ditching the system file that contains the log of DP's AU diagnoses (see paragraph 2 above) I didn't even have to reinstall - it just agreed to give BRASS a second look, and this time found nothing untoward.
So now it works exactly as expected, and on my 2.4gHz Powermac pro, even the big processor hit has never affected the recording/sequencing process.
Now I'd like the chance to complete the Arturia user survey with the program working properly. I think the program is excellent, but that it could use further development - streamlining in some areas and expansion in others. Even so, an investment I no longer regret, and a program I would now recommend to other musicians.