<<The tricky part with the Westy bottle is you'd need to elevate and
invert the 5-gallon supply tank above the Westy bottle, then figure out
how to connect the two. I don't know if the pressure in the supply tank
is sufficient to fill the Westy bottle, or if additional pressure is
needed.>>
--
Mike "Rocket J Squirrel" Elliott

Here ya go.
As soon as the pressure equalizes between the two tanks the fill up will
cease.

So you'll have a half of a BBQ tank and whatever the Westy tank took.
The brakes on the fill up are now set.

You'll need a pump to overcome this  equalization phenomena.

Now your spending the $2900.00