It's a great guest space for EcoBBBers.
Here's the beefed up floor bed, some insulation in the bottom of the cavity, the beginnins of some mass on top of that (broken glass... 'just to put it to use, instead of in a landfill), and the radiant tubing.
Now that bed has the sand on top of it for ever more mass. I believe it was a couple cubic yards of sand altogether.
The skeletal solar heating system guts. The pressure tank was already replaced by a small reservoir to make this a "drain back system". More detailed photos forthcoming...
it's turned out great!