It is difficult to deal with line-breaking after the fact, as with breqn.
In order to do a proper job, Maxima needs to know the width of the
eventual display line
(e.g. are you preparing a paper with 2-column format or 1? Or are you
making a powerpoint
slide?)
It is not so difficult to do this formatting if you are not using all
the bells and whistles of TeX
(or Maxima) . After all, the regular display does it, to some extent,
though of course not
everyone likes it.
A typesetter version of the (Macsyma) display was written in the 1970's
by John
Foderaro for his MS project, "Photot", but for the "eqn" typesetter, on
UNIX. I think that
wxmaxima must do something similar.
My feeling is that doing optimal line-breaking for use in a formal
static presentation
generally requires human intervention.