I worked in the Browning Gunsmith Shop in the late '70's, spent a lot of time working on BAR's. Never had an occasion to do any trigger work on one though. The sear/hammer surfaces could probably be lightly stoned to reduce friction and thereby the perceived trigger pull. Because of the trigger groups design and the fact the rifle is a semi-auto I would only have this done by a competent gunsmith who is familiar with BAR's. I am not aware of any aftermarket triggers available for this rifle. I would not go any lower than 3# weight of pull.
I've had a few people ask me to work on various BAR triggers and I just won't do it..
Like you said, a bit of stoning can help, but I won't touch anything else.. Just too much liability on those - as it is on most semi-auto hunting rifles., FWIW.