But what would be the advantage of that over a simple USB flash drive?
Again, any speed increase that you would get from using a filesystem on RAM would be completely nullified by being on the USB bus, and now you are even talking about having your non-volatile OS storage and volatile "scratch space" on the same port, effectively halving throughput.
The idea of "taking the RAM with you" as a security measure will be completely ineffective, as the system itself obviously couldn't use the volatile storage in the USB device as actual RAM. So the system would still be susceptible to physical RAM attacks (I.E. hitting the RAM with cold blast and extracting data). Even if there was some software implementation like ReadyBoost in play, that has nothing to do with the fact that the CPU needs to execute code from physical RAM sitting on the motherboard. So your data will eventually have to make it's way outside of your little USB computer.