what are good spots to take a higher variance line in a MTT?

I assume playing more (and weaker) implied odds hands in position when a bad player is already in the pot, or in the blinds.

Perhaps being more willing to shove thin value on hands that look like busted draws? Perhaps willing to jam flops/turns with TPGK that look like trying to get fold equity with a draw?

In the big picture, you should be trying to reduce (or at least manage) variance at all times.

In the smaller picture, you should look to reduce variance against players whom you feel you can outplay, and you should try to increase the variance faced by players whom you feel are capable of outplaying you.

This approach requires that you are honest with yourself about your ability; since 90% of the guys in any tournament believe that they are in the top 20% of the field in terms of skill, this can difficult.

In the scenario you describe, you should seek to reduce variance against the guy you feel is a bad player; in the normal run of play you will beat him, so variance becomes his friend.