It is apparently possible to use the dashboard vents, not with the parking heater but the auxiliary (engine block) heater that it seems we have fitted as standard on the Beaches (check for a Webasto or Eberspacher sticker on or around the drivers door). There is a remote kit to turn this aux heater on. There has been talk of using this heater as an alternative to a dedicated cabin/living area heater. Its really intended to warm up the engine though so you would be heating the block and engine oil unnecessarily when camped up. As we have this aux heater already its a simpler install for the parking heater. Usually the fuel tank needs to be dropped to fit another fuel pump but with the aux heater the supply can be tee'd into that pipework.
Interesting that the latest Beaches appear to be coming with the vents in the B pillar. Think this thread originator looked into changing the B pillar trim to include the vents but there was no obvious path from the vents back to the heater unit. Hopefully there now is.
Good question about why people are choosing Webasto over Eberspacher, they seem pretty much on a par to me. Both in performance & cost.