More editable solder mask

Today it is possible only to increase/decrease the
clearance area between a pin in the soldermask
(selectin Show Soldremask on the Screen Tab), but
sometimes it is important to make some work on the
solder mask. Like, if I have a TO220 series regulator,
like 7805 mounted layered with the board, it is a good
idea to draw a great polygon below him to make heat
dissipation, but to do it the area below him must not
have soldermask, so the component get directly in
contact with these ground area. Today, I make it by
editing the postscript output in sketch, but I cant do
it with the gerber output.
The idea is to draw rectangules, polygons, and maybe
lines too, that makes a XOR with the soldermak.
To make it, it would be nice to make the soldermask
translucent, like with the option in the pcb rc file,
that makes the opposite side translucent.
It is important to have lines to edit the soldermask,
because sometimes it is needed to remove the soldermask
from above a line, so in the hot air process the line
receives some solder in it, and increases the current
capabilty.