I have use BL to push IIS metabase username changes and password changes. What I did was navigate, via BL, to the location in the metabase where the password was being stored. I then added to the depot as a BL Package. Then you adjust the value accordingly and deploy the package. I was using 7.1 at the time, so it should work for you as well.
I used BL to provision new web sites, and configure the app pool to run with a specific account. May vary a little if you're doing site pw instead of app pool pw. It took a while to get it right, so I'm trying to remember the hoops we had to jump through. We had to update metabase properties 9026 AppPoolCommand and 1012 ServerCommand after setting new values for WAMUserName and WAMUserpass.
See MS doc for what the values on these props do