Do you think it’s technically feasible to split the .plot files if the harvester is told where to look for over multiple disks for a whole file? Or if the file is split in a certain way, with the regular harvester software?
It’s technically possible. Ultimately the chia application is going to make some system call to get needed data for different parts of the plot, you could provide your own replacement implementation that looks for different parts of the plot on different disks, there might be an even higher level in the chia application where you can plug in your own implementation of seeking parts of the plot file.
If it was me looking to do it, I’d prefer something at the filesystem level, because you don’t have to think about any chia-specific details, and the work has probably already been done for you. You wouldn’t need to replot to move to RAID, but you may need somewhere to hold the plots temporarily, and that might not be feasible.
Here’s a post on this forum where someone tries to solve the same problem on Windows, i.e. making a virtual volume out of the unused parts of already-plotted drives: