"Hot Gravel" - another alternative to substrate coils?

I am new to this list (heck I am new to the world of aquatic plants AND aquarium fish). However, I have been following this list via the archives at the www site. In particular, I am keenly interested in the recent threads on the pros and cons of different DIY substrate heating systems and proposed R/UGF schemes for simulating UG heating coils.

I read in the July AFM about a pre-heated water-based UG heating device that appears to be cheap and easy to DIY. I am curious if it has been discussed in this list (I didn't see it in the archives). The idea of pumping hot water to the substrate from a separate heating chamber (at 10F degrees or so higher than water in the main tank) via PVC pipes sounds simple and logical, and you can utilize many of the parts you likely already own, such as a spare heater and a powerhead. Any comments on this design (details in July AFM)?

Before I read the AFM article, I was wondering about the possibility of pumping (slow reverse flow) pre-heated water into an existing UGF setup. Would that simulate (even to a limited extend) the heat convection currents of the commercial heating coils? This would be even easier to do than the AFM article method.