Jack, I have not, but that could probably work.
I guess my question though, would be why not go ahead and put in the posts for the inner fence as well.
With only the bailing wire holding the position of the wire on the inside, I would imagine you would end up spacing your plastic posts closer to the corners. That being the case, it seems that any cost benefit of not bying more t-posts would be lost, being they are about $4.50, while a plastic step-in, in the $2 range.
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As for your earlier ? about running insulated wire, I imagine you could effectively do so, but I'm not sure how long a reel of insulated wire you can get... and it seems its kinda pricey, if I remember correctly.
Another option would be 8ft tall t-posts with the wire ran up-top, out of way from a passing deer, tractor, or man. This is how my grandfather used to carry electric from one side of a field to another. But there again 8 ft t-posts are not real cheap either, these days.