The memory limit has been increased:
23.0.61 (9/19/2017)
7. Changed: Increased the cap for memory playback track loading to 2GB on 32-bit and 16GB on 64-bit.
But I don't know if that means SACD ISO's will fit in memory.
Looks like it was changed again in MC24.
24.0.45 (7/26/2018)
8. NEW: The maximum memory playback size allocated is expanded to 4 GB from 1 GB on 64-bit builds.
As far as I know, right now it's 1GB for the 32-bit build, 4GB for the 64-bit build. Going higher than this would likely either require a manual setting to set a limit or have MC detect how much RAM is in the system, how much is being used by the system and have it use up to like 60% remaining or something like that. And even then it doesn't seem like a good idea to me to go beyond 4GB. What if a system only has 4GB or even 8GB if the limit was increased again and you load a VERY large file (nearly maxing out the limit or going beyond it, which is also as much RAM as the system has)... would the system become bogged down, slow or even unstable because of all memory being used? Just seems to me there needs to be a sensible limit, and 4GB seems good (since most systems have more than this these days). But right now it's files larger than 4GB (or specifically large SACD ISOs) that might have issues, so again, I'd recommend just disabling memory playback as a workaround. For another workaround I suppose you could use a tool (e.g. ISO2DSF) to extract the music from the SACD ISO into individual files (I'd recommend DSF) to workaround that limitation. That's what I do and my SACD ISO derived DSF files work fine with memory playback.
Or I guess a couple ideas would be that JRiver could automatically disable memory playback for ISO files in general and/or set it to disable memory playback for the file when attempting to load a file that's over 4GB. It would actually make sense to as this isn't the first time this has been mentioned, likely won't be the last either.