I've got some non-soup playlists in a hierarchy like so:

Electronic -> Vocal -> Fast
Electronic -> Vocal -> Slow
Electronic -> Instrumental -> Fast
Electronic -> Instrumental -> Slow

is there any way of constructing a playlist based on an advanced search, which combines the two "slow" playlists together and the two "fast" playlists together? The only solution I've come up with so far is to add "slow" or "fast" to the comments field and do it that way, but is there a more elegant solution?

Any reason you need to do it via a search? You could create a playlist that contains the two slow playlists as children.

what, you mean create playlist FAST SONGS, and copy + paste the two fast playlists underneath? The problem with that is they wouldn't stay in sync with the original electronic-vocal-fast and electronic-instrumental-fast when I added to them, would they? (My original post said non-playlist soups - it should have said non-soup playlists. I've changed it now).

they wouldn't stay in sync with the original electronic-vocal-fast and electronic-instrumental-fast when I added to them, would they?

Yes, they would, as long as you copy the playlists and not the tracks themselves.

Like Bitt said, playlist links are "live". More details on how the playlists work is here.