A new version of MaximumMPD is now in the iOS App store that supports downloading Album Art via UPnP. If you choose it you will be prompted to select an available UPnP Media Server. The Poly's UPnP's Media Server linked to the SD card should be listed as "Chord Poly:<Poly Name> [Server]".
The App will search for Album Art URL's based on the Album Name and Artist Name. You will have to ensure you have Cover.jpg files in the albums directory.
One problem I have found with The UPnP software that Chord uses (MiniDLNA) is that search fails to find Album or Artist names that contain "&". This seems specific to MiniDLNA and I have tried other UPnP servers and searching with "&" characters works fine. Not sure how I can work around that.
Also I have made an Android early access version of MaximumMPD available in the Google Play store. It should be mostly feature compatible with the iOS version (No Now Playing support). I personally don't find it performs as well as the iOS version especially with browsing Albums. My main MPD db is around 12000 songs, 1000 Albums and 380 Artists to give some perspective.
Here is the Google Play link to install the Android version. As it is early access you can't search via Google Play for it.
https://play.google.com/store/apps/details?id=com.maximummpd