Bookmarking in this case will just involve a map between the a file and a timestamp within it; then you just tell MPD to play the track starting at the timestamp.
Technically speaking every player can implement it (and JRiver did it). The OP was speaking of a Pi, and to my bad I didn't know JRiver runs in Linux too.
Then, can JRiver be controlled via some sort of web api? Or do you need to be in front of the computer on which it runs?
P.S.
the Okto, AFAIK has a Pi inside.
I usually use JRiver through BubbleUPNP, but they do have their own apps - JRremote and Gizmo. JRemote and Gizmo work great and have great interfaces, but cannot connect to streaming services such as Qobuz, which is why I use Bubble (the owner refuses to do so because of past problems of some kind). I also use Bubble to stream to Moode on the Pi (in my Okto) since it's headless anyway. Any application (supposedly) can connect to JRiver via DLNA.
JRiver can also be set up to automatically load and run in the background (without the use of a monitor) every time the computer is started.
Note that I do NOT and have never used JRiver for VIDEO and/or VIDEO streaming, but for video purposes, JRiver does do that (I believe).
As far as the Pi in Okto, it can be purchased both way - with and without. I purchased without because when I got mine, they dropped the availability because the shortage of Pi units - plus I already had a Pi4 that I was not doing anything with since I had it to piddle with IanCanada's reclocker and i2s boards (which I no longer use). I believe they have made the option available again though now that Pi4s are more plentiful.