Yes the heat output from an AGA is enormous, prehaps the least enery efficient appliance ever invented.
We put cement board between the AGA and the units to protect them from the heat.
I love AGA's even though they are inefficient, I did a blog on them if anyones interested my AGA blog
Regarding heat transference from appliances. My company is involved in the building and fitting out of canal boats which invariably involves the installation of solid fuel stoves. The regs in this area have changed recently and whilst not specifically applicable to your application there are obviously some interesting parallels. It was found that even with the then standard minimum 100mm space between appliance and bulkhead the heat transference was enough to cause scorching and occasionally fire, even with tiles and fireboard applied to the timber bulkhead surface. This was compounded in boats used in a residential capacity because the appliances typically operated for prolonged aggregate periods. The most efficient modification was found to be stepping the fireboard and tiles off the bulkhead by 3/4 - 1 inch (on battens) leaving a small gap top and bottom to enable air flow around this area. With a little clever design this is almost invisible and the difference in heat transference through the bulkhead is phenomenal. I'm sure someone of your talent would easily be able to work this type of modification into your designs if you are concerned that this may be a problem for you in some installation situations.
I truly hope you find this helpful, if you would like any more advice I can put you in touch with experts who know far more than I do. I appreciate not all of what we do will be relevant to your application but some may be helpful.