Google announced today that ad group-level mobile bids will be coming to enhanced campaigns. The added functionality will be available to all advertisers in May.
This is a fairly substantial modification. Currently mobile bids can only be set at the campaign level in enhanced campaigns. It is unclear if this change is in reaction to advertiser feedback or if this was a planned change.
API access for mobile bids at the ad group level will be available in early May. Look for that announcement to come on the Google Ads developer blog.