Unfortunately, there is no way around this rule so you will have to at least book a main cabin fare in order to be eligible for an upgrade. (rewardexpert.com)
You can earn 5,000 miles for qualifying economy class fares and 10,000 miles for premium cabin fares. (rewardexpert.com)
If all they wanted to do was reward premium business, they could have increased bonus miles for premium cabin fares. (viewfromthewing.boardingarea.com)