A few suggestions:
Plan for cables to somehow die and be replaceable, as weird things can happen and its great if spares are there or you can re-thread your cables without major surgery. If you have to take it all apart at any stage to change over a cable you will curse yourself that you didn't make it simple to swap a dead cable in and out.
Run USB3 cables to your mount (two) and run them into a powered 7 port (3 x USB2, 4 * USB3) selectable on/off hub (around $50 from office works - see link below).
Sticky velcro-tape some 4 or 6 power board (selectable on / off) two thirds up the side of your pier (great for Camera power adapters, and focuser power transformers).
Plan to see if you could run all USB cables to your powered hub; meaning at most only one or two USB3 cables run to your PC.
Make sure all your cables are if possible double insulated if they may fall anywhere wet (where dew condenses) - prevent corrosion.
Consider running a Cat6 cable to your pier - if you ever think you might but a networked device to put on your rig.
Stay away from USB2 extender cables unless you can be absolutely sure dew will never corrode anything - I find the extenders really prone to corrode easily so now I use 4metre USB3 cable straight to a powered USB3/USB2 7 port Mbeat hub from Officeworks.
http://www.officeworks.com.au/shop/o...b-hub-awm43hub