I understand that you don't want to do it on the weekend, so that the servers are flooded. But you're asking players who are still progressing to give up one of their valuable raid days to test a boss.
Our guild is currently 6/13HM raiding only 3 nights a week. One of them being Tuesday.
A lot of guilds raid on Tuesday, only guilds that aren't progressing anymore (13/13) are going to give up a raiding day to test on a Tuesday. I guess if I was in Paragon or Vodka blizz would cater to me too.
Can we please get some better days to experience these bosses?