Could you use something like this in a jail?
https://github.com/xat/castnow
I don't have any experience of it, but might give it a spin if I get some time.
Oooh nice idea, thank you. That would work great with video. Not so ideal for slide shows as I imagine controlling it from a phone at the sofa would be far from smooth. Plus the non-techies using it are going to have a fun time. That would especially good for movies so ill look into it for that.
My current working plan is that I have a VM Lubuntu running (surprisingly smooth) on 512mb of RAM. I wanted to have the desktop app to upload photos to google photos but there is no linux support from google! Tired a few linux alternatives but TLDR they don't work with google photos properly. I can use google chrome and use a local gallery plugin to view files of the NAS and cast directly which is wonderfully smooth compared to accessing the NAS from my phone and then casting (unusably slow). The problem is that FreeNAS VMs seem to be stuck at 1024x768. That makes sense for most jobs as I imagine that gives performance gains but you can't view content on it :/ It comes out as 4:3 on the TV. I can't find a way of outputting chrome cast at a different display resolution (which seems kinda dumb).
[UPDATE: With some use, I can report Lubuntu works great in a VM with little RAM. If you don't fullscreen google chrome it works pretty well, not perfect but acceptable, the only proper way I can think of is using an Android TV and cut out chromecast all together.]
I think the next thing I will try is emulating the pFolio android app (which allows uploading to google photos) or regular google photos in an emulator for chrome. Not sure if it will run on a lightweight Lubuntu VM only time will tell. Having FreeNAS synced to google photos would make for a VERY nice chromecast experience... if its possible.
[UPDATE: This didn't work, all the Linux apps I found don't do the unlimited photo sync and most don't even do automated sync]