I have worked on plenty of Marzocchi's for riding buds, they are pretty easy to work on if you are mechanicly inclined and pay attention to how things come apart.
I've installed X-heavy coils in all my Marzocchi forks.... While changing only the coils will provide better support for heavier riders, that is not hte only peice to the puzzle.
If you are requiring heavier coils you will probably also need to change the viscosity of the oil for dampening and rebound control as well.
Per the Marzoccji website:
"Forks using the SSV Non-Adjustable System can change their rebound damping with oil viscosities. Stock oil is 7.5 weight, so changing to a lighter oil (Example: 5 wt.) will increase the rebound speed. Heavier riders using stiffer springs may want to adjust their rebound speed with a heavier weight oil. Be sure only to use a high quality motorcycle fork oil like Marzocchi's Factory Fork Oil. Refer to the Setup Chart for recommended settings."
The setup chart should be found in:
http://www.marzocchi.com/system/2604/2004-bomber-en.pdf
(I can't open PDF's on this system right now.)
I run a 15wt oil and X-Heavy coils and it is just about perfect except on really cold rides (eg. below 20°F) For winter riding I use a blend of 50% 7.5wt : 50% 14wt.