Drawing rectangles / outlines over object faces

Hello,

I'm a long time flash developer, and now learning unity by making a doodle project. It's a turn based strategy game. I managed to print the game map out of blocks and did some camera controls.

here:
http://aukyr.com/games/m0ar_soil

However,
I could not figured out a way to draw a grid over this map. I know I could just draw the grid over tile blocks, but I don't want the grid line thickness to change when map is zoomed in or out (mouse wheel on my demo)

If you are coming from flash/as3 like me, I'm looking for a substitute to this:

mySprite.graphics.lineStyle(1, 0x000000, 1, true);
mySprite.graphics.drawRect(0, 0, 100, 100);

Generally I'm asking some advice for doing things like these: (circles, hexs rects on terrains)
http://dl.dropbox.com/u/27318600/Temp/unityForum/rectOnMapTile.png
http://media.pcgamer.com/files/2010/06/CIVILIZATION-V-E3-2010-GunshipVinfantry-590x3591.jpg
http://t2.gstatic.com/images?q=tbn:ANd9GcRc_p_K36i3FcipKRtKDP5eLHPwRLpfvA_42J2S8wEfGB9EVvbQyg
http://www.cyberstratege.com/magazine/wp-content/uploads/2010/10/civilization-5-header_33.jpg

Another solution I can think of is to put flat transparent objects containing the grid lines slightly above each tile. But wouldn't this cause glitches, artifacts?

As I'm building myself a set of solutions lists cheat sheets for unity like I did for flash years ago while learning it, I'll be glad to hear "how you would do" these.

Thank you.