Big (pardon the pun) subject there George,
there was a rule which was deleted back in the sixeties that essentially meant all drivers under 10 stone ( about 60 kilos) had to carry weight in/under their sulky seat to make up the difference.
I think to handicap harness horses, ( whilst there is no doubt that weight is a factor) by weights is an unproven but certainly researchable option.
Being a little fat bastard myself I would look forward to finally getting some sort of advantage out of that but I seriously don't like my chances.
If you look at the issue scientifically, when you start measuring the affect of weight on energy when being drawn on modern roller/ball bearings nthat are commonplace these days, the weight factor is ALMOST not worth talking about.