I see in the docs that PushButtonField.LAYOUT_ICON_ONLY is int 2.
Did you try that ?

Duh! Now why didn't I think of that? ROFL!!!! Of course I did. The behavior is really goofy. When I was testing in Visual Studio, these lines were required to actually update the icon on the button:

Cmd.Layout = PushbuttonField.LAYOUT_ICON_ONLY;
Cmd.ProportionalIcon = true;
Cmd.BorderWidth = 0;
Cmd.Visibility = BaseField.VISIBLE;

So, after the icon got updated, I started removing lines of code one at a time to see which one was required and after I had commented all of them out, the icon continued to get updated. What I finally did was to translate these lines of code to VFP like so:

loCmd.Layout = 2
loCmd.ProportionalIcon = .T.
loCmd.BorderWidth = 0
loCmd.Visibility = 0

And my VFP code started working.

Sometimes I would rather be lucky than good....