Hi drm870,
[quote]Well, talked with the 'rents. They indicated that, when the time comes to buy, they’d be willing to give the Dreamfoam 12-in-1 (with an extra firm layer in place of the soft) a try. The relatively low price and the customizability won them over, as they perceived it to be a lower risk as a result. (The lack of of those aspects in the Brooklyn Bedding and Tuft & Needle mattresses turned them off from them, so I hope the 12-in-1 works.)[/quote].
That’s good news 
You can see my thoughts about a solid surface support system in post #10 here. It would probably be fine in terms of providing suitable support for the mattress but there would be some additional risk because the plywood wouldn’t provide any ventilation or airflow under the mattress. It would probably be fine in terms of meeting the warranty criteria as well (although if the mattress did develop any mold or mildew issues then it probably wouldn’t be covered by a warranty) but I would check with Dreamfoam just to make sure.
Phoenix