The problem with "that dog goes last" is that I assume it means "last in the class". There's another class that starts just a few minutes later in agility.
My other concern (and maybe it isn't valid, I'm not really around intact females much) is - is there actual leakage? I'm assuming so, since people use panties. We have more and more trials in the area being held on indoor turf (usually soccer fields that aren't used in the summer, so they rent them), and they can be very, very protective of what is allowed on them. Most have a "no liquids other than water" rule, and some have a "no food other than dog treats" rule.
I think what would be a good option would be for it to be made optional for clubs, as in UKI. It would allow clubs/trials to really see what the financial benefit may (or may not) be. If they draw in more people by allowing dogs in heat vs losing people who don't want to deal with their dogs being distracted by it, I think more clubs would start allowing it. And it might let people who have some concerns see if it is actually going to be a big deal. If it isn't, more people would be supporting the idea.