I have been tinkering with the process of using mixplorer to get the sync app working, and believe I have refined the process somewhat, at least for Android phones:
Required Apps: Mixplorer Silver (This is available for download from the Google Play Store for ~5 USD)
The Musicbee Wifi Sync app found in either this post
OR the Google Play store.
Mixplorer is essentially granting the Wifi Sync app access to the android file structure since it can't access it directly for whatever reason.
1. Install both Mixplorer and the MusicBee Wifi Sync App.
2. Open Mixplorer. It will ask for permission to access the file system. Grant it.
3. In Mixplorer, tap on the three dots in the upper right. Then tap on settings, then tap on Components. Scroll down to the bottom of this list and turn "Document Provider" on. Once done, restart your phone.
4. When complete, you can open MusicBee Wifi Sync. The app will ask for admin access. You can go ahead and grant this.
5. The first time you open the app, it will prompt you for a folder to use. What Musicbee is looking for is for you to select the root folder of the device.
This is why attempting to create a new folder and selecting that throws an error in the Wifi Sync App - it needs to use either the root device folder OR an SD card. And, of course, Android throws a fit about "privacy" if you try to use the root folder as is. But...
6. With document provider activated in Mixplorer, there will be a new sandwich menu available in the upper left of the folder select screen. Open this and select "Internal Storage", then "Use This Folder." It will ask for permission to access files again; grant it. As noted above, instead of accessing the root folder directly, we are now accessing the root folder through Mixplorer.
7. You should now be able to select either preview or sync now and sync your files.
A potential issue with Ephoras's walkthrough above is that he says you should add the music folder to home, and then select that when WifiSync asks for a folder. Depending on how you defined where MusicBee puts the files when initially syncing the phone (remember, phones need to be initially synced using a USB cable first; the default is /Music) then this can create a situation where the path to your music on your phone will be Root/Music/Music. Some users may prefer this (Root/Music/Music, Root/Music/Playlists, etc) but selecting the root folder should allow you to put your music wherever you want by manually changing the paths for the device settings in MusicBee itself. It will also allow you to use the default Playlists/Podcasts/Audiobooks folders that MusicBee assigned by default in the device settings page.
This is really two approaches to doing the same thing, but this one does not require you to add any files to home/favorites within Mixplorer itself and more closely replicates how the configuration used to be.EDIT:
after some extra testing, I've confirmed that this method actually also works with the default version of the WifiSync app available from the Google Play store. So for those of you worried about sideloading the APK, you don't have to worry anymore.