I have open-ended tags stored in the Grouping field of my (iTunes) mp3 collection. I also have information written in the Comments field. A lot of my music is also BPM'd. How can I search by/sort by/access these fields in iOS? The Music app and the 3rd party apps I've looked at so far don't provide this.
Music Stats provides some of what you're looking for, I think. It doesn't show all ID3 tags, but it does provide a few of them.