Here’s what we have done. (With United)
You can’t do this online, but have to call customer service. When you find saver economy fares, call and say you want to pay for saver business fare but with a confirmed seat in economy, and ask to be waitlisted if and when business saver fares become available. You are not asking to be put on an upgrade list.
When you receive your confirmation, be sure the business miles have been deducted. If you aren’t confirmed in business before checking you are added to the upgrade list and since you paid for your business seats (with mikes) you are prioritized over people on the upgrade list.
I also continue your look for business saver fares, and if I find something, I call to change flights. If you don’t end up in business class, your excess mikes are refunded.
I have a link to a web site describing the process for United will add post when I’m on my computer.