After reading Broquea's blog on Plex Media Server and IPv6 I decided to have a look at the server and the Apps.
First the server. Enabling IPv6 was fairly painless. However after ticking the Enable server support for IPv6 I did find it necessary to reboot the server.
I tested access via both the servers routed /64 address and also the localhost address [::1]:32400/web
The localhost address was recognised as being on the local network and allowed me to connect successfully without requiring a logon.
When accessing via the /64 address literal I was required to log in however so I looked at the white-listing feature.
AFAICT this only supports IPv4 address/netmask combinations. Therefore there's no way to white-list an IPv6 address
(please correct me if you know a way)
However this did give me a way to check if my hostname was connecting over IPv6 as I white-listed my public IPv4 address to allow me to connect without signing in first.
Using Web browser on both PC and Ipad Mini I was able to connect via IPv6 (although IOS/Chrome happy eyeballs did initially connect me via IPv4, this changed after I managed a successful v6 connection with Safari).
Next I had a look at the Apps
I used a Sony Xperia Z and an Apple Ipad Mini.
Both Android and IOS Apps provide an interface to allow manual entry of an IP address. However:
Plex for Android does not allow the entry of : or letters, so it is only able to enter an IPv4 address
Plex for IOS appears to allow manual addition of an IPv6 address but it would not connect. Being as I had connected via Browser over IPv6 literal I can only conclude that the App does not actually understand IPv6 addresses.
plex.tv and myplex still don't support IPv6 either.
Finally I looked at Plex For Home Theatre on my PC
Again while this supported a manual address entry, it is only able to add an IPv4 address.
While Plex media server itself supports IPv6, direct access to the server is ONLY available via Web browser. Therefore true IPv6 support is extremely limited.
Comments would be welcome.
Edit - corrected minor error regaring connection via [::1]