The question is, is this style of wiring still permissible? It violates
the guideline of having no more spurs per circuit than there are actual
outlets directly on it, but I can't find anything in the regs that
explicitly forbids it.

It is likely to be a perfectly OK design.

Forget about any "more spurs than sockets" rules of thumb. They are just
guidelines for mentally challenged electricians. The important things to
determine a

(a) That the end of the spurs have sufficiently low earth loop impedence for
a rapid disconnect on an earthlive fault. (Plus voltage drop calcs if
required.)

(b) That the ring itself is reasonably balanced, so that sockets and spur
connections are evenly placed around the rings, with sockets likely to have
heavy loads either moved off ring (preferably) or distributed around the
ring.

Christian.