Just verified that there is no insulation. Would laying 3/4" of foam
on top of slab (R4 I think), then plywood with pex be a good
approach? Do I need the aluminum in the plywood to make it work?

Radiant floor heat works by "radiating" the heat into the living space. The
more surface area, the better. When the tubing is placed within the
concrete, the entire slab becomes the radiator. Otherwise, you'll most
likely need the aluminum plates to take the heat from the PEX and radiate
it into the room. I doubt you would get enough heat from the bare tubes to
be efficient.

If you insist on hydronic floor heat, there are companies that make panels
with the aluminum plates and tubing channels ready to go. One source is:

http://www.warmzone.com/hydronic-rad...oor-heatin.asp

My other option is to use hydronic baseboard heaters.

If you really want radiant, you might want to consider electric radiant.
Very low profile, and easy to add as a retrofit. The warmzone site above
also offers some of those too.

Of course, the simplest option would be to add in-wall electric heaters
like those made by Cadet or King Electric. We heat our entire house using
King Electrics "Pic-A-Watt" heaters. They're quiet, efficient, and we can
zone each room for only the heat we need.

Anthony