Harpo, i've done some tinkering with Goafan77's directions. and in order to get the independent colonies like with the slider in the map designer, simply set the "colonize chance" field in the text editor to 1. Also, if you are using templates, the planet owner and colonize chance fields for the template itself have to be set to "RandomMilitia" and "1" respectively. and the owner is case sensitive. just tried it on a map I've been working on, and it works beautifully, all i have to say is, I just gained to ability to put some serious roadblocks on the map.
p.s. if you can't find the fields, here's a sample piece of the code, i've highlighted the pertinent fields and you can find the specifications for the template(s) at the very bottom of the map's code:
pos [ 548 , 296 ]
template "claimed Colony"