Thanks for all the recommendations. I believe my plan may be hindered by the "you cannot get a "100W speaker" to put all 100W into a single frequency" thought from mettelus.
I have set-up a couple of 100W tweeters attached to separate 2-3 foot lengths of PCV (to help guide the noise only to the target problem dogs), driven by a cheap 200W car amp. But in my initial tests, the dogs don't seem to notice. I could buy more speaker wire and move the speakers closer--that may be the least expensive option. I don't know if there are inexpensive transducers that would squeal out a tone around 20KHz loud enough to reach these dogs (about 50 yards).
Also, in my initial test, the audio I exported and sent to my iPod for playback contained a lot of periodic clicks--not sure if this came from an exporting artifact (44.1k sample rate to a wav file) or from the limitations of the iPod, but I didn't hear it directly from my DAW.
Thanks again for the help!