Dude, writing in any time is easier than it sounds, you just have to write around the measure, which is why bands like DT or The Faceless or even Tool end up sounding generic, because you can't really get a good flow going with an original riff if you're trying to focus it around a certain sig...
My idea behind writing has always been 'write the signature after I write my riff'...my riffs sometimes end up being some crazy shit like 7/13 (pretty sure we nailed one down like that) but I have a great skill in circling it around and making it fluid...it just ends up coming out that way...I think it's because I didn't have formal guitar lessons before, so my mind has always ran wild with riffs...