Seems like while gnomevfs supports accessing files on ssh servers like local ones the c# bindings banshee is using don't.
My guess (it is only a guess right now) would be that if you used fusefs to make the android device appear in /mount like an ordinary device would banshee would see this device. And if this device contained the .is_media_device file identifying it as an MP3 player it would actually use the device.
Let's give it a try (I have set up password-less authentification with my device using ssh keys, my local username is gunter, my username on my android device is root, and since this device isn't rooted ssh is running there on the non-standard port 2222)
sudo mkdir /media/Sulla.home
sudo chown gunter:gunter /media/Sulla.home/
sudo chmod a+r /etc/fuse.conf
...add the following line to /etc/fstab:
email@example.com:/mnt/sdcard /media/Sulla.home fuse user,allow_other,port=2222
A final command at the command-line:
Now the device looks like an ordinary mounted device. Let's hope banshee will be convinced by this.
If this does not work, a possible workaround is to use the FolderSync extension with what we just mounted.