Possible to force all disk activity to RAM?

I don't believe you'll be able to accomplish this. Windows is designed around the concept that the system partition is always available, and your experiment with the RAM overlay shows it's not designed for what you have in mind.

Instead, change the system partition. Use a non-hard-drive boot partition -- a LiveCD or USB key. You can certainly disable Windows access to non-system partitions, so moving the system partition off of hard drives will let you power down any attached hard drives.

Update:
After reviewing the materials on EWF, yes, this is possible.

I still think this is a case of not-what-it's-designed-to-do, but...

The EWF you used in WinXP came from Embedded XP. The Win-7 analogue is Windows Embedded Standard 2009, and does include the Enhanced Write Filter. I don't know if you'll be able to shoehorn EWF into Win-7, but you'll have better luck with the modern version than the old Embedded XP version.

It looks like you want "EWF RAM Reg mode", which places your EWF configuration data in the system Registry, and doesn't require a separate partition on the drive. The other modes (EWF Disk and EWF RAM modes) at minimum require a separate partition to operate.

You will want to review the techdoc on EWF Performance Considerations, particularly disabling last-access timestamps on NTFS, disabling System Restore, and changing the size of the pagefile (although it seems preferable to disable it, if possible).

Consider designing your own run-time image as your OS base. By doing so you can eliminate components of Win-7 that may be problematic in an embedded, static configuration (which is essentially what you're describing). You may also want to use a checked build, if available, at least until you get things working as expected.

Even this is, at best, educated guesswork. This method seems to be what a LiveCD uses to accomplish its magic, so you might have more luck looking into those. Even if a LiveCD is not your end-goal, the methods they use to build a system image may assist your endeavors. Best of luck!

If money is no object, you could replace the noisy HDD with an SSD. I have a MacBook with SSD and it is totally silent (and faster).