Looks like the latest and beta repos for MC23 is up, but stable isn't yet.
I can't recall if the current MC23 build is in the latest repo or not, but it is in the beta repo. You *can* run both the latest and beta repos at the same time (it'll allow updates to betas if desired).
First, import the gpg key if you haven't already;
wget -q "http://dist.jriver.com/mediacenter@jriver.com.gpg.key" -O- | sudo apt-key add -
Second, add the latest repo for ARM;
sudo wget http://dist.jriver.com/latest/mediacenter/mediacenter23native.list -O /etc/apt/sources.list.d/mediacenter23.list
Finally if you want to, also add the beta repo for ARM:
sudo wget http://dist.jriver.com/beta/mediacenter/mediacenter23native.list -O /etc/apt/sources.list.d/mediacenter23beta.list
After that just run
sudo apt-get update && sudo apt-get install mediacenter23 and that should do it.