Re Windows ReadyBost, is not really additional, it's a faster copy (or most used parts of it), the swap system is still active, but writes to disk are delayed / separate work and reads are directed to flash stick. You can unplug it at any time and your system will still be stable. And that page is customer facing, telling them about swap is like explaining what is a bit
Coming back to XBMC4Xbox, it first needs a virtual memory system

And I think even if that's implemented with I would assume some considerable amount of work (all memory allocations would need to be "virtualized" to write to memory and/or swap) and do the work in the background with physical and virtual memory allocations, swapping when needed, reading back when needed. Most of us we take it for granted, but it's not such an easy feat (each OS has a whole memory management system under the covers), and even if that's finally implemented and working, there's no guarantee that the performance of the virtual memory (disk) will be fast enough to guarantee any performance gain, other than being able to run larger programs or skins or whatever, but quite possibly, slower. So instead of getting a crash, you could get a sluggish UI, responding back in 30 second.
Implementing a ReadyBoost like feature is also another thing on top of it - even with ReadyBoost you need to have a fast flash stick AND a fast interface. As I recall the comments on the forums, the USB interface to XBMC4Xbox was quite slow, so I would say that would be a no go from the start.