I haven't had the pleasure of doing this on the A10 yet but what always worked on the Norton Commando I had for 20 years was to prop the head 1/2" or so clear of the barrels (the Norton has the rocker box integral with the head) then use the long screwdriver to get the pushrod ends into the rockers whilst gently waggling the rocker to make sure it was seated. Obviously this means the valve end of the rockers is a long way from contact but by gently lowering the head everything lines up.
The rocker covers on the Norton were much smaller than the BSA ones so I would have thought a similar procedure may work. I can see that the comb affair could cause problems with the gasket and any sealant used.
