Ummm
Interesting setup...
I do have a nagging thought...
If you have a laptop running the FM Radio app for your poolside, can you not just use that machine to host a remote control interface of some kind?
Either, use some media player software that provides it's own web access?
VLC has a built in web server that is quite powerful and gives mobile compatible pages.
Kodi also has a choice of web interfaces that are very powerful.
(Both of which I've had working with a little HTTP Request app on my Android devices, mainly to double check the http commands, but for simple transport / source / volume / query commands it's often the quickest way to interact.
FYI, this even works for sending ad-hoc events to an OpenRemote controller.
For example, "press" a button on an OR panel that is linked to a macro, or set a fader value.
https://play.google.com/store/apps/details?id=ch.rmy.android.http_shortcuts )
However, using OpenRemote to control either of these media players is also possible, BUT I've only been able to get basic transport, volume and "play a playlist" or single file / source commands working. (I'm really not a programmer)
So far I've found that the inbuilt webservers in these media players are far better at handling the track selection (etc) than OpenRemote.
You could....
Create commands in OpenRemote to handle basic transport control or pre-set source selection, then use a webview element to call up the media library from a URL?
Obviously, I don't know which bit of software you're using for the FM Radio dongle, so can't comment there.
However, I read somewhere that TVHeadend might be able to access FM Radio dongles (with some back end tweaks).
Alternatively, a cheap (<£30) DVB-T/T2 {Or DVB-S / DVB-C} dongle should work quite well and give you a good selection of local stations. (Assuming you are lucky enough to have digital broadcast coverage)
But that's a really confusing way of getting some music playing outside.
FM USB Dongle / DVB-T/T2 dongle into TVHeadend, accessed via Kodi client, controlled by a web server, which in turn takes Json commands from OpenRemote.
Spin off thought, you could create a separate set of panels within your cinema OR controller that gives you control over your poolside music?
{If your home looks anything like my imagination has created, I'm fairly sure you'd get a long line of volunteers willing to 'pop in' to help you}
You could go really wild and use a single board machine to run a TVHeadend server inside you house / palace, then another single board machine running a headless version of VLC by the pool to pull audio streams from the TVHeadend server?
Or you could fit a full on Kodi machine in your cinema and use a headless device by the pool to pull audio from Kodi.....
Oh the options are endless....
Good luck :-)