[chrismorse]

I have done a few dryer runs.
The best thing to use is 4 inch "SD" pvc pipe. It is thin wall and readily available at most ace hardware stores.
Use large radius elbows or build up the turns with short sections of pipe and 1/8th bend elbows.
Keep the turns to a minimum.
Keep the pipe constantly sloping downhill, or you will have water ponding in the pipe, which attracts and holds lint.
If it is a long run, insulate the pipe to keep all of the moisture from condensing in the pipe.
Leave one or two joints loose, (not glued up), so you can do the clean out.
Make sure the pipe is well and frequently supported, or things will come apart or you will get ponding.
Use the semi rigid aluminum flex to get from the dryer to the pipe run. Make these turns as gentle as possible. You can sometimes find an aluminum elbow that will fit onto the back of the dryer, which leaves you with just one 90 degree turn to go through the floor.

The booster fan idea is also good, but simpler is better.
I like your idea of the hatch in the floor to access it from the bottom.
Be carefull with "Ts", they are often a very tight turn.

HTH,
chris