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I was looking for a method of adding support to this STL manually. It's an effector with integrated airflow based onth Jimustanguitar's design, but adapted for the new ball-joint-arms.

http://www.thingiverse.com/thing:1284511
Of course the overhang of the fitting-tubes will fail when printed with no support. If I set support in KISS (or Slic3r) the airflow will be obstructed with internal support that you cannot get out of the way. So I came across Meshmixer and thought that would be the solution. This plant a few pillars below these overhangs and print. But I cannot seem to get the hang of that manual support adding. I keep getting things like this:
Anyone knows how to deal with this? Or is there an easier method for adding supports manually?

I'm not familiar with the model, but many slicers (including Slic3r) have the option to produce supports 'everwhere' or for parts of the model that need just need support from the bed. It looks like 'bed only' support might accomplish what your looking for. Also, one of the big advantages to Simplify3D is that you have very fine grained control over support (you can manually add and remove supports via the UI) - but S3D is expensive, and (in my experience) takes a ton of tweaking to get good prints.

Yes, I have been looking in Kiss for that option, but can't seem to find it. A bit like I can't find rhe option to set rhw amount of priming loops. Does anyone know Kiss does have an option for support only from the bed up?

EDIT:
I've set the Support Z-Roof option to 1mm. That prevents supports from getting generated above 1mm, but seems to go forward on the supports that were already being generated from below 1mm. So that basically is what I need I Think. Will give it a shot!

Nevertheless, I would like to learn about experiences with Meshmixer. It seems to generate way smarter supports.

My son printed that using Cura and stock settings- he's been using it for a few months now.

I'm going to assume he didn't have any issue since he didn't ask for help and only printed it once.