Just wondering what peoples results are using a A Herculiner / Rhino
Liner Type Product on the belly of the van to prevent rust expansion.

I used Herculiner on my Gas tank because I heard GoWesty was using it
to coat the out lower section of their vans for rust protection. I
have to say its been on the tank for a 9 months no rust present at
all.  The stuff Dries Hard Like a ROCK!
http://www.herculiner.com/other_uses.html

Here are posts from another site stating how good it is:
http://www.mgexperience.net/archive/herculiner_used_as_undercoat/673308
http://forums.off-road.com/jeep-mid-sized/158582-went-got-some-spray-truck-bed-liner-today.html

Now in some cases you may not want it to be hard as a rock, I assume
that some parts of the van's body may need to flex and it might be
best if you had some flexiblity, no?

Not sure what is best. All I know the stuff under my van is peeling of
everywhere, especially in the wheel wells.

Duplicolor also makes something but from one of the forum messages
above they said it did not last long and was not very good.

Sears had some stuff that seemed interesting:
  "MIRACLE" PAINT PERMANENT RUST AND CORROSION INHIBITOR $50 1 QT
http://auto.sears.com/MIRACLE-PAINT-PERMANENT-RUST-AND-CORROSION-INHIBITOR/Product.htm?catalogId=10623&N=600025414&productId=2000773

PROFESSIONAL RUSTPROOFING CHEMICAL  - $41.99 4 Pack
The same chemical used by a major U.S. auto manufacturer
Specially formulated to be used under the car as well as on the inner
panels, rocker panels, doors, hood and trunk
Seeps into the tiniest cracks and corners to assure complete coverage
and protection
One-step, high-performance chemical is easily applied with
rustproofing kit (sold separately). Thoroughly tested by being exposed
to salt spray for over 2000 hous without failure. High solid content
assures better conding and protection. Chemical actually displaces
moisture on the surface to which it is applied.
*Won't dry out, always tays soft and pliable. Suitable for all
climates in North America. Meets government and military
specifications for rustproofing materials. Includes 4 one-liter
containers of rustproofing chemical (enough for one full-size car) and
25 body plugs for sealing panel access holes.

http://auto.sears.com/PROFESSIONAL-RUSTPROOFING-CHEMICAL/Product.htm?catalogId=10623&N=600025414&productId=2000777