"Is the signal going to be blocked when the luggage rack is loaded with stuff ?"

Metallic objects like coleman stoves or tent poles or dense wood objects like a load of firewood could interfere with the signal, but anything else should be okay.  Try to keep metal and dense wood things off to the side away from the antenna and you should be fine.

"has anyone hard wired the 12v plug  ...I presume it could be hard wired to operate only when the key is on"

The vanagon doesn't have an accessory or switched on position using the key like most other vehicles.  I don't have a plug-and-play unit like yourself in the van, but my Sirius tuner and Sirius Kenwood headunit are wired to get power when the key is in the ignition.  I took the cover off of the steering wheel column and taped into the wire that goes hot when the key is placed in the ignition that I think is used to activate the seatbelt alert.  This provides a switched on power source that goes dead when you remove the key.  The Bentley manual told me the color wire to use, and it tested out okay.   Email me if you need me to look up the color.

BTW, weather conditions are not supposed to affect satellite radio like they can affect satellite TV.  I finally took my wife's wireless plug and play unit to another town that happened to have a local station broadcasting on the same frequency that I had set the Sirius unit to use.  That caused a lot of interference.  I switched the Sirius unit to another frequency and the interference went away.   This issue does not affect the non-wireless units.

caverat