Thanks to all who responded.  For those interested, I've decided to go with
the following Thule setup:

1 x LB 78 - 78" load bar set (2 per set)
1 x #387  - high foot gutter set (4 per set)
1 x #544  - 4 lock set
2 x #579  - canoe carrier

I decided against screwing artificial raingutters into the pop-top because
of what Thule's Fit Guide says re Westfalias:

#18. The 542 Artificial Rain Gutter Brackets require drilling mounting
holes in the roof. Thule recommends that a qualified auto body shop install
the brackets.
#26. Due to the weight carrying capacity of this roof top Thule recommends
carrying only skis or two bikes.

The #387 high feet will clear the pop-top when attached to the Vanagon's
rain gutters, and with this setup I can carry two heavy canoes.  The only
downside is that I'll have to remove the rear bar when I want to pop the
top.

Rich Jones
'89 Westfalia