I haven't found any frameless solution, since there is not way to access
an unnamed base document. Actually there is one, but it's
unidirectional. If you create pop-up window with window.open() and give
it some name, you can access it from the base form, but now the other
way around.

The calling window is
window.opener
you can call functions of the window.opener page anytime you like. I call a
function of the window.opener passing values of the pop-up window I want to
set/change - the window.opener then saves and displays the values after
closing the pop-up

Alan