My Kona Sutra has disc brakes but it also has the proper eyelets down there to attach and I've been able to put two different racks on there. The one I had before I just zip-tied to the seat post. It worked great but I recently bought a different rack that has long arms that hit the braze-ons underneath the seat post. You can check the picture here.

I do have the issue you mentioned though with it being too close to the seat post and so I can't really attach an under-the-seat bag there. I use a trunk bag every now and then but typically just stick to a pannier or a shopping basket on the trunk. If you think the rack I just bought might work for you I can give you the details about it.