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I think this is a good starting point. After trying with radials of
different lengths, I tried the MP-1 with the steel tape measure
counterpoise I used with the ATX. I managed to get good SWR on all
bands 10 thru 20 and no RF on the feeder. Haven't come up with a
solution for the lower bands yet. though.

I echo your warning about the risks of adjusting for low SWR, though.
On any band, you can move either the coil or adjust the counterpoise
to get the best SWR. However, I found that you can be moving the coil
away from the point at which signals are loudest, in order to get the
lowest SWR. You need to keep an ear on the signal level as well as
the SWR, and try to pick a coil setting/counterpoise length combo
that gives the best results for both. The MP-1 is not an easy antenna
to tune, for that reason.

To be honest, I don't think the MP-1 is an ideal antenna for use with
the FT-817. It is better suited to the K2, which has a built in
tuner, so you can peak for maximum noise and then just hit the Tune
button. Trying to adjust the MP-1 to get a 1:1 into 50 ohms seems to
be the root cause of all the problems. Which is understandable when
you read the antenna books which state that a base loaded whip is
anything but 50 ohms...

Julian, G4ILO