Teri-Lee, the joist structure must meet the customary L/360 deflection requirement, but the spacing limitation has to do with the subflooring rather than the joist structure.
The published method requires a single layer of nominal 3/4" plywood for a subfloor over 16" joist centers. It's quite possible that adding a second layer of nominal 5/8ths" or thicker exterior glue plywood subflooring would make it work at 24" spacing, but there's no published method specifying that. Doesn't indicate it wouldn't work, nor that it isn't done, just means no one spent the money required to get it to the level of acceptance by the TCNA Handbook committee. If I wanted to use that method of heating installation for myself, I'd not hesitate to do it over the improved subflooring. I'd be using a membrane on top even if I met the criteria for the published method. See my warranty information below.
What Alex is suggesting would be less expensive, but is also limited to 16" joist spacing and simply filling over the hydronics with deck mud would not meet industry standards, either. You would need to fill to the tops of the tubes with deck mud, then add a cleavage membrane and then a minimum of 1 1/4 inches of deck mud with welded wire reinforcement in the vertical center. By the time you did all that, after installing the necessary layer of subflooring, you'd be getting pretty tall. The SLC would be a good bit lower, requiring plastic lath and a minimum of 1/2" thickness above the hydronics (unless the SLC manufacturer requires more). And a good bit more costly.
My opinion; worth price charged.
