yes. Use the vanilla Novalith (PLANETMODULE_TECHORBITALCANNON.entity) ability for anti-planet bombardment and the Kostura (PLANETMODULE_PHASEORBITALCANNON.entity) ability for anti-ship/structure bombardment.

I am unsure of what the actual names of the needed abilities are, so I provided the names of the structures that use them. The abilities should be at the top (or bottom) of the entity files.

Also, is the Mardukas made (as in modelled/textured&tangent-map set) fully? If so, can I use it (it looks SO AWESOME).

Also, you aren't the 1st one to think up such an idea. I too have had this concept in my mind; I guess that you are just the first person to mention it.

  Interesting drawings, and i would say yes, although my experience with importing models into the game is nil, others on the forums have quite a bit of experience at it. The difficulty with that as i see it, is having the experience with one of the modeling programs that is compatible with the .mesh format that IronClad uses for the game. But it is certainly workable. 

With regards to the planetary cannon that bombards from one gravity well to another, I would say it is certainly done with all the superweapons of the races, the Korture Cannon of the Vasari, the weapon the Tec use and the Advent Culture Cannon, all of these fire across the entire system to attack planets at a distance. So your idea is certainly workable. 

I would suggest first off looking at the weapons entity files, or if you are unfamiliar with that, to ask someone who has a bit of modding experience in that area. Perhaps they can suggest something, or point you to something that has already been done in a mod that you can look at and see if you can tailor it more to what you want.

Take care and good luck!!!  Some very interesting ideas!! 

-Teal