It kind of works that way already. With 64GB of RAM the OS will use most of it to cache the disk. Very soon you will have most of the files you use in the cache and you will have the effect of a RAM drive. And the OS will of course save the changes to disk.
a RAM drive is a bit obsolete except if you are running on a very small computer that boot off a USB stick.
Oh dear. How long will it take El Capitan to go to sleep/shutdown with 64GBs!? My puny 8GB MacBook Pro is sometimes taking over a minute to go to sleep since installing EC. The times were way shorter with Yosemite (like 15-20secs or less after about 45secs with Mavericks). I'm guessing it's the hibernate thing, which I haven't tried turning off since installing EC (never could get it to stop writing the hibernate file or stop waking up when the power cord was plugged in/unplugged with Mavericks/Yosemite).