I am in the paint-and-coatings manufacturing business, although I have avoided fire-retardant products simply because there are too many folks already offering such products.  I prefer to offer unique products.

This post caught my attention because, having recently developed a surface-stabilization treatment for Shou Sugi Ban that eliminates the flaking and charcoal contact-transfer,  in the course of that I had to learn something about this wood-treatment. 

The essence of it is that it develops a char of essentially charcoal on the surface.The charcoal thickness may be less than a millimeter, **or may be many millimeters**, and there we have an opportunity to do some thermal engineering.

The charcoal itself is combustible, although far less than plain wood.  The charcoal is porous, (and if thick-enough offers a degree of thermal insulation) which suggests that some commercially available fire retardant treatment might be a candidate to impregnate that charcoal layer and thus give a surface that would certainly be more fire-resistant than chemical-impregnated cellulosic material, namely wood.

I googled your referenced  specification and found this

http://osfm.fire.ca.gov/downlo...

and it clearly states that appropriately fire-retardant-treated wood may meet its requirements, because it gives that as an option.  If plain wood so treated can meet its requirements, I suggest that wood bearing a layer of fire-retardant-treated-charcoal ought to more easily meet that specification.

If you have a client that wants to support your development work, the above might offer a Path Forward.

http://nakamotoforestry.com/

http://charredwood.com/

ask these guys, almost the only ones offering it in the states.

There are two other suppliers of Shou Sugi Ban; Delta Millworks https://deltamillworks.com/sho... in Texas and  reSAWN Timber Co https://resawntimberco.com/sho.... in Pennsylvania, these latter three offering the Made In America option; there maybe others but I only see those four suppliers on a casual Google search. 

You can also make your own if you have a contractor you trust and the time / money / interest to do enough mock-ups. If you can find a coating that you can convince your AHJ to allow (using an engineering judgement or similar) that could work.

I'm waiting for Hardiplank to introduce man-made Shou Sugi Ban panels.