Does Gleemax allow you to choose an invalid target for a spell or ability? If you have it on the field and the other player plays and spell that say choose 2 creatures you control and place a +1/+1 token on them. Can you choose a creature you control or do you have to choose one of their creatures?
Keep in mind Silver-Bordered games always require some play-group-specific interpretation of the rules.
That being said, Gleemax changes who selects the targets, but it does not change anything about the legality of choosing targets. Player B selecting a creature they control for player A's ability that must target 'a creature you control' is illegal because the qualifier for the target refers to player A, so it effectively means 'a creature player A controls'.