The idea would be that Maxima sends "labeled output" to TeXmacs:
each subexpression which you might want to select is given a label.
TeXmacs may send this label back when the expression is selected,
The problem I see with labeling input is one has no idea what the user
might select. He selects a form on the texmacs side with the mouse.
It might be terms 20-24 of a polynomial 100 terms long. Or that might
be in the numerator or exponent or matrix entry. As you parse it into
the internal TexMacs tree formula, you could just record as you start
a tree what character index you were at in the input string -- or else
perhaps better have a subsequent parse pass compute the range in case
the user moused some box ... In this latter method there is no extra
storage associated either on the maxima or texmacs side, just a cost
of running thru the display code a second time.
I once constructed something like texmacs where I did the display myself,
and this is what I did.
william