This limit could be overcome by using spacecraft to increase
the baseline, radio interferometry rather than optical methods, or a
combination thereof. I don't know what progress has been made with
such methods. This might allow direct measurement of trig parallax out
to around 3 x 10^8 light years.
It should be noted, though, that such measurements would not
necessarily give more "real" distances than other techniques - cluster
parallax, apparent luminosities, angular sizes, &c, though various
assumptions have to be made for all of these techniques and some care
is needed in relating determinations of distances by different methods.
Froehliche Weihnachten,
George Murphy