Yes of course, this is pretty much a gesture of love for the competition that gives so much back to the platform. As long as the rules are followed to enter the competition, you are welcome to do it.

That being said, donated posts don't have to be about music even, if you have a post that you feel comfortable adding @openmic as a beneficiary, you can do it. I will listen for the tag and do my best to give it some extra love.