I see what you mean. There isn't a feature to tell Makerbot's slicer to do something like "only print the base layer with fat lines". You'll have to do a little experimenting with the raft settings to get Makerbot's slicer to do what you have in mind. I think it's possible, but you'll have to find the right values for the relevant settings to pull it off.

You're going in the right direction for what you want to do though. Besides the "Base Layers" section, further down the list, you may want to try "0" for "Number of Surface Layer Shells", "Number of Surface Layers", and "Number of Interface Layers".

Come to think of it though, I heard folks have only recently developed a slicer that lets you do variable width printing during the printing process.

Well good luck and have fun.