In practice, intercepting from above is not a problem, but it is taught as a no no because of the possibility of picking up a false glideslope.
I'm no expert on this, but seem to remember something about reflections of the glideslope beam forming repetitions of itself above the one that your looking for. Thus, if you manage to pick up a false glide, you will prob end up at the right place, but having flown a steeper approach. It is for this reason that you validate the glideslope on the way down, check altitude at 4nm etc...
It wasn't mentioned above, but you can fly a CDA with a verical speed of as little as just 100ft/min (as long as your below the transition level, otherwise the 500ft/min expectation from ATC applies). Rocking along at 200kts, 100ft/min almost approximates level flight, not quite, but almost.