Well, they're on...
I wound up boring out the Halcyon bar-end bracket to pass a larger M8 bolt (which is what RE uses to attach the bar weights), then overbored the outer face of it out enough so that the head of the M8 weight cap screw fit down about halfway to flush in the end of the bracket cap...
I bought a pair of M8x60 socket head cap screws to replace the OEM bar weight bolts. The new cap screws are long enough to pass through the mirror brackets and the weights, and thread into the bars. Next, I took the first section of the Halcyon bar-end mounting wedges (the one with the stepped area that fits into the bar-end mounting bracket) and cut it flush with the widest diameter. This creates a flat, round face to friction-fit against the outer face of the bar weight. (I'll have to shoot some photos of this, as I suspect this description ain't makin' no sense.) Finally, I assembled the new, longer 60mm M8 cap screw through the Halcyon bar-end bracket, through the aluminum thingummy from the wedges, and through the bar weight, adding a 2mm thick x 18mm dia. rubber washer between the weight and the mirror bracket stuff for added friction and grabbiosity. All of this then threads into the handlebar end, in the existing weight-mount bracket.
Net result: Perfect. While I will stipulate the #835 mirrors are very small, they hardly vibrate at all, plus the mass of the mirror assembly seems to have added to the bar weight mass, providing additional anti-vibration damping. And the bike just looks *mean* with 3" bar-end mirrors... I call it a win. And it only took me about an hour, once I had it all sussed & the fasteners acquired.
Stay tuned for photos, as I know this post may well be useless without any.