How would you use this as a test? It seems you'd end up with highly variable results, depending upon who they drew. One guy draws an MMA guy who is primarily a striker, and the next guy gets a BJJ blackbelt. Very different tests for those two.
You can set the rules for striking only, grappling only, stand up only, all of the above. Beyond that I wouldn't care if someone is a BJJ blackbelt or not. The testing wouldn't be based on "winning." If the student winds then great. I'm more concern about a student being able to read the opponent, read the situation, picking the "best fit" technique to deal with what the opponent does. I can use your same question of " what if the guy is a BJJ blackbelt?" and apply it to a street situation. What if the student gets into a physical conflict with a guy who he doesn't know is a BJJ blackbelt? The one thing I never do personally when I spar is ask my opponent's skill level. As a martial artist that trains for the purpose of being able to physical use the stuff I train. It's important that I'm able to determine that on my own, by the way the guy moves or position himself. I need to be understand what I'm facing, so that's something I would like for my students to be able to do.
If I was going to grade a student it will be on things such as ability to use techniques, read the situation, read the appoint, and exploit the opponent, and defend against the opponent. If I see that the student is constantly getting surprised by the opponent then I wouldn't be happy. This stuff would be unacceptable for any student who I would consider being on an intermediate level. Some of it would be unacceptable for a beginner level and I'm basing this on the students I've trained and how I corrected them on similar things, for example, that bouncing, hands down, reaching low to block kicks, not dealing with the lead hand, reaching to block vs. bringing the arms towards our own head to protect our heads, These are things that I currently teach get on my beginners and I lecture the intermediate students about for doing.
I expect my intermediate students to have a good read of what is front of them. They should be able to recognize where the real opportunities are and the mistakes that their opponents are making. At the intermediate level they should be analyzing and working out possible solutions. In sparring, winning isn't a big deal nor focus for me, because if a person can't do any of the things I stated then they won't win any way regardless of how well they perform kata or kung fu forms. They won't win regardless of how many Karate or Kung Fu people they fight.
The chances that any of us is going to get into a street fight with someone who does the same fighting system that we train is almost a 0% chance. This why I believe students have to have those good analytic skills and understanding that they are facing. MMA students are the best way to test that because they are so random from student to student and they fight more like people in the street than they do TMA students do in the school or at competitions. Just keep in mind that for me it's not about winning. There are other more important foundations that students need to have so that they will have a chance to win.