can I use a flap on a 28 wire wheel ?
Yes.
You need rim liners not flaps.
http://www.snydersantiqueauto.com/354
FLap will add a couple ply to sidewalls.Today's tire can use that. I run them in my 26 wire wheels.
Flaps are much thicker than rim liners and provide better protection for the spoke end/nipple not poking a hole in the tube.
I have used Flaps for over 30 yrs. and have great luck on not having flats. Less chance pinching a tube when using flaps.
Thank you. --will use them