You didn't manage to convert the first markup to the correct syntax.
glisslinescript = \markup{
"\\embeddedps{ 0.2 setlinewidth 1 6 moveto 3 -8 lineto stroke}"
\halign #1 { \italic gliss. } }
Also, to use these definitions, you have to specify to LilyPond if you
want the markup above the note: d^\glisslinescript
or below: d_\glisslinescript
or if you want LilyPond itself to determine: d-\glisslinescript,
just as for all other text scripts.
However, I guess you will have to adjust the parameters in the
Postscript code to get what you want.
/Mats