Rockbox 4.0 appears to have solved this and solved it big.
Before using 4.0, I could never get my iPod Mini 2G with CF-SD adapter and 256GB card to burn less than 12-15% of battery per hour, resulting in a max of 7 hours playback.
I've been letting it play all morning and now it's only using 3-4% per hour. On track for at least 24 hours of playback, an improvement of over 3x.
This is HUGE and I think needs more recognition than just a bullet point in the release notes.
Good job and major thanks, Rockbox devs.