Hi, Kat. I don't think a *thin* layer of sphag on top of the pot will hinder the getting of air to the roots. A *thin* layer. This is because good new uncompacted sphag has a remarkable ability to hold air *and* moisture at the same time.
Of course, if you put on some stinky old sphag on real thick, that might be another matter
I've used a thin layer of sphag *sometimes* for both catts and oncids where the plants needed new roots that were forming at the top of the pot: The new roots aim themselves right at the sphag and grow right into it. They *love* it, just spread out and grow and grow in it.
Now you need to remember this is different from a whole lot of sphag at the bottom of a plastic pot, which wouldn't work the same way. But certainly no harm to the plant, and often a benefit from the thin layer of sphag.
Don't know if it would be good for paphs or zygos, as I don't grow them.
I just wanted to comment on phals in plastic pots: Bob Gordon, in *Culture of the Phalaenopsis Orchid* recommends making extra holes in plastic pots for better root aeration. Most pots come with only 4 holes and this isn't enough. I use a 3/8" soldering iron, and hold my nose while I burn new holes in the pot bottom. A couple of holes at the *side* of the pot bottom would be good, too.
Don't know if you do that already, but it's worth it in peace of mind about root aeration. Also, I use a net pot or a few peanuts in the center of the rootball of phals to make sure the center of the roots doesn't get waterlogged.
Hope this helps.