The new "subset" booleans coming with the next v3 beta can help with this kind of stuff. It will let you get it done with just one boolean union since you can limit which faces are used to actually generate cuts. Some info on it here: http://moi3d.com/forum/index.php?webtag=MOI&msg=6663.1

The way it would help with this particular case here is that after you create a rib like this (it may be a good idea to try and reduce the rib to its minimal overall size like this, it just makes for a smaller amount of overlapping surfaces to be resolved), with it just sticking all the way through a ways:



Then you need to set up a face selection on the main object that leaves out the back side of the object, the easiest way in this case to do that is to go select the back faces like this:



And then invert the selection. Then with those back faces not selected a boolean union between that main object and the rib will generate this type of result:



It will basically ignore the back side of the object when doing the union, if those back side faces were not selected.

- Michael