I don't think it's bidah sis. It makes sense. I mean just as in a normal nikah a woman can state that her husband has to divorce her if he takes another wife, similarly in a Misayr marriage a woman is waiving some of her rights.

hang on lemme give u an example...

a lonely/old/divorcee/widow woman marries into polygamy for companionship. She knows the man has another wife and kids and they need him more. So she can waive her rights of him spending equal amounts of time between her and the other wife etc... maybe she'd like his company only now and then. that way they are both clear as to what is expected in the marriage.

Doesn't that make sense?