That's something we may consider for a future release if there's enough demand.
It would need to work both ways to be truly useful: files copied from the source folder at mount, and changed files copied back at dismount or shutdown. One-way copying is the easy half, but it also means anything you create or edit on the RAM volume is lost on reboot, which is rarely what people want.
Worth noting that you can get most of this today: create the volume with an image file and enable auto-save. The image loads at mount and is saved at shutdown or dismount, optionally at intervals too, and saves are differential - only the parts that changed are written, so a large file that changed slightly doesn't cost a full rewrite. That's faster and gentler on SSDs than a full copy.
The genuine advantage of the folder-based approach is that the source stays as ordinary files you can browse, edit and back up while the volume isn't mounted, rather than being sealed inside an image. That's a fair point in its favour, and it's the part we'd be looking at - thanks for the suggestion!