Bob, wouldn't it be very easy for a dog to avoid a single foot-high
wire? One of my dogs would probably hop over it (she believes in
shortcuts) and never learn that it was "hot" at all! Same possible
problem with other dogs in the neighborhood.

Why not use the same equipment -- electric fence fenceposts and wire --
to run the IF perimiter wire? I don't know enough about the electronics
to know if you could just substitute electric fencing wire, but if not,
big spools of genuine IF wire can no doubt be purchased and running them
above ground on elecric fence posts would certainly work.

The dog would have the same visual reminder (and if the dog has been in
an invisible fence for a long time, he's probably boundary trained and
could be taught the new boundaries easily; as you point out, if he's
boundary trained he'll never get buzzed, or at least he shouldn't if
he's properly introduced to the new fence). It wouldn't keep other dogs
out, but that is a problem with invisible fencing in general and appears
to be one that the owners don't have an issue with.
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Only know that there is no spork.