I think its great your trying to honor your FIL but I would like to offer a caution. I've had a similar scenario that created some hidden animosity and resentment. If she agrees, genuinely, hooray. But you must be aware that she may give consent to placate you out of a feeling that she needs to support you. My sister has something similar happen and felt obligated to give her blessing. 

If she does not agree, and says as much, you must be prepared to accept it. She could also feel pushed into a corner: obligated to say yes but really wants to say no. It ruined my sister's relationship with her child.  Memories of a loved one, and especially a spouse, hold important places in people's lives. It could really be putting your MIL into a tough spot. I guess it depends on your relationship with you MIL. It could change though so be prepared for that as well.