Honestly, I would. The way things are going down here in my country (Brazil) I will get infected eventually, at least by participating in challenge trials it would help other people.
That's an difficult and potentially eternal question - some people volunteer to go to space or cross the ocean on a canoe because to them its worth the risk. Some do it for the money. Some are just mad or obsessed.
Back in my time at uni, volunteers had to be unpaid to ensure they do it kf their own free will. Its unclear which is better / worse.
A lot of people who take trips and meet friends and family (me included) instead of staying home, basically said that they will take the risk of being infected than losing part of life.
Given that we have seen that the poorest are the most at risk of dying from the disease, I'm not sure that argument even holds any more.
Sadly, it's this obsession with straw man arguments about ethics and equity that's cost the West a million lives in the last year.