I felt that the tentacle was the most important part to figure out or the model would be a flop. I went ahead with the rubber core/plastic knuckle design but I still needed a way to to pose the tentacle. While the rubber core would make it super-flexible, it wouldn't hold a position. I decided to insert a brass rod through the length of the tentacle much like they do for stop motion puppets. The rubber would give it flexibility while the brass rod would hold the shape.
The parts then have to be exported as STL files for the printer. STL files describe just the surface geometry of the piece which the printer uses to slice up and build the model. I had to export parts of the tentacle as separate STL files based on what material they will be printed with. I output one STL with the rubber core and suckers and another with the plastic knuckles.
The finished tentacle with armature wire installed. It worked out pretty well. The knuckles have fused to the rubber and stay put like they should. Take a look at slide show below for a detailed account of the printing process.
UPDATE: just added this video which covers most of what's below.
I also feel that the armature wire was thicker than it needed to be and contributed to the rubber tearing toward the tip where it was the thinnest. Overall a great start and good learning experience.
Having done this test, I feel confident about modeling everything correctly. It also made me realized how important the workflow is with this type of project. If you boole something out of order or take it into ZBrush at the wrong time it can be disastrous. I had to rebuild the tentacle due to improper booling on the first version. If I had not caught it, the print would have failed and cost me money. I also had to be careful of keeping all the geometry watertight and avoid manifold edges. But with all of this in mind and a properly organized workflow it should work out fine.