Here's an update on what may work for me-- my ProTaper SE arrived today and I got a chance to measure it and try some things.
I have the OEM handguards and their hardware---which involves a shorter M6 bolt than the one which came stock on the bike. There is only about an inch of length left at the end of the bolt. However, the adjusting/tightening nut mechanism on that hardware is definitely too thick (0.67 in) to fit inside the Protaper bar (0.54 iN), as was expected.
I plan on replacing that OEM adjusting nut mechanism, using a short section (half inch or so) of 1/4 inch ID fuel hose that I found lying around (originally sourced at Pep Boys parts counter for a car). The OD of this fuel hose is .50 inch, so it just fits inside the Protaper bar and the idea is that when compressed as the screw is tightened, the rubber fuel hose will bind up against the inside of the bar and hold the bar end weight in place.
I created a new adjusting nut mechanism as follows: The short piece of fuel hose will be sandwiched between two 6mm steel washers. Before adding the 6mm nut at the innermost end, I am also adding a 1/4" ID rubber/neoprene washer between the nut and the steel washer----an idea I got in this thread from another post, so that the nut adjusts properly when the bolt is tightened. so, the sequence from outside in is: M6 steel washer, fuel hose, M6 steel washer, 1/4" rubber washer, and at the very end M6 nut. The steel washers are not strictly needed but my thought is that they will help compress the section of fuel hose, hopefully without binding up the adjusting nut and making it all impossible to remove.
The only new parts required for this mod are short piece of fuel hose, one M6 nut, two M6 steel washers, and one 1/4"ID rubber washer. It seems to secure the bar end weight but I have not yet installed and tested on the bike.