hi,
The 5k6 is not in series with the zener, its from the zener Anode [28V end] to 0V, so its across the 28V to 0V.
A load of 5.6K is about 5mA across 28V.
OK.?
You can use diodes or a zener or both, but as Eric pointed out you need to have a load of at least some resistance on the output or you will not see the true voltage drop. This is because diodes need current flowing though them in order to drop voltage; at least some current. 5ma sounds like a good start. You can then measure the output voltage and get a more realistic reading.
Another option you might consider is a LM317L (TO-92, SOIC or 6 bump SMD). It's rated at 100ma & 40V max, so with 28V @ 20ma it would be fine with 4V of overhead. Just add a cap and 2 resistors and that's it for under a buck.
OH yes, the ever popular and illustrious LM317. A very good idea. So much for so little parts count. It requires so little support parts i've used it in applications that require voltage adjustment (via pot) even though the output regulation didnt have to be that good. Just have to watch power dissipation, but yes with only 4v overhead it shouldnt be too much of a problem. I would bet no heat sink required at less than 100mw.