You are correct, rear curtain sync is a very old technique that long predates modern flashes and will not be hampered just because you're using a manual flash.

That being said, though most are, not ALL manual flashes are well suited for performing a rear curtain sync. Because the camera fires the strobes just before it closes the curtain, the electronic lag plus the strobe duration need to be short enough to complete before the curtain starts to close. Some flashes on the lower end of the cost spectrum might not be up to the task.

EDIT

Based on Michael Clark's comment below, I did a little additional research. I use mostly Nikon gear, which uses the center pin (or sync cable) to fire the rear curtain sync and so was not aware of this. Apparently Canon equipment uses a digital signal and as such won't work with a manual flash (or with wired studio strobes, which boggles my mind).

My basic advice will remain the same though, a manual flash does not automatically preclude rear curtain sync, but not all equipment is compatible, so always check the manuals and, if possible, test the equipment before you buy.