Heating a waterbed can be the largest cost associated with it, and a
bit of insulation in the design could save a LOT of money over the life
of the frame. I also really don't like the typical smaller pedestal
design, our bed has the drawer units the same dimensions as the frame
around the mattress).

Despite arguments about bumping toes ( I never have), my guess is that
the real reason for the pedestal design is that its cheaper, or used
some existing standard sized drawer units. Making the base drawer
portion the same size as the mattress frame, looks MUCH more like
furniture to me, and provides a bit more storage etc. Think about it,
maybe draw a picture, and see if it isn't more appealling to you as
well.

Water has a HUGE heat capacity, the lower the amount of insulation
between you and the mattress, the smaller the comfortable temperature
range of the water. Neutral to the skin is around 91 F in the water,
and maybe 72 F for air in the room. One degree of change in the water
is like maybe 5 degrees change in the air depending on air motions and
blankets etc.

The builder can't do anything about heat loss off the top, thats pad
and sheets and blankets. I like some heat warming the stuff in the
drawers, so I am suggesting insulation for the sides of the mattress
frame, and reducing the airflow if not insulating the outside of the
drawer base instead of just insulations directly below the mattress
support, except maybe something directly below the heater pad to kind
of reflect up.

I know just enough to be dangerous, so wiser heads correct me. Instead
of the typical 1" solid wood frame with a padded cap, how about making
a sandwich of 1/2" PB or PW, 1" insulation, 1/2" PB or PW, with a solid
wood top and/or bottom plate? Maybe make the drawer faces larger than
the openings so that they close flush to the surface, and generally
avoiding gaps etc. for air to flow through the bottom.