I don't have any experience with doing so on a Mac, but it works with something like a ThinkPad X40 or X41, for example, where it's quite a common upgrade.
Using a modern mSATA SSD with a 1.8" IDE converter, as shown in the picture, is the only feasible way to upgrade a ThinkPad X40/X41, as they used 1.8" IDE drives, which are impossible to find these days.
You'll have to look into whether TRIM will work on older versions of MacOSX, but SSDs are pretty cheap these days.