Obviously when doing this, you set up two colonies on a body. One being the actual colony to be used and another is simply a "target" site somewhere on the other side of the body to let the dust drift across and thus not blasting the population out of existence.
Does this actually work? Can you bombard one colony on a planet while leaving the other untouched?
If so, do dust and radiation carry over?
How about mass drivers?
It works to an extent with some fudging, first up I can't find out how you were able to use a laser to target one of your own colonies? I can't even see my own colonies in the combat screen. As such I had to do a quick test with a work around.
1. Design a new race with identical properties to your main race (thus costin zero RP).
2. Using the new race in the F9 screen add a colony to your chosen body that for example already has a colony of your main race.
3. An unfortunate step but easily handwaved with some RP (radioactive waste etc), you need to have some form of installations or such on this new colony as it needs to have a thermal or EM signature for detection.
4. Give this new colony independance so they effectivley become a player controlled new empire
5. Have a TG detect this "new" race then target the colony on the combat screen (ensuring "show hostile only" is unchecked).
6. FIRE!
Doing the above causes an orbital bombardment as per normal, in my test run i used a heavy cruiser with a vollet of laser fire to inflict 208 points of damage to the colony. I had only SM'd in a single factory to test as a thermal signature so obviously this was totally destroyed. This also resulted in a dust/radiation level of 208 being produced and a temperature change of -2 degrees, the dust/radiation effect was felt across the entire planet. So when I switch back to my main empire I find my main race colony on this planet now are suffering from a dust/radiation level of 208 and a temperature change of -2 degrees also.
The same could be achieved easier with mass drivers I would think as you could skip the first four steps. Just setup a colony as per steps 1 & 2 but ensure a frieghter is present to unload/load a single mass driver to the colony and allow you to throw spare minerals at it.
If you were able to target one of your own colonies with a beam weapon as per your OP then this entire process becomes simplified, and I would definitley like to know how you did that as it could be used for short term aid with terraforming and as a good RP tool.