"Once the light hit's a surface that's it! That surface is what it is, and will have exactly the same effect it would have if were already that intensity."

I think you're wrong, Len - how else can you explain the result in post 101?. True Ambience doesn't work properly with light that has had shadow-casting disabled*.

"...and it lead to the realisation that 100% with TA is a definate no-go!

Maybe a no-go with your Pro-Render requirements, but essential to proper True Ambience operation.

" - you'd break it."

I think it's already broken.

*Bryce creates its low intensity shadows by converting a proportion of the light that strikes a surface into non shadow-casting light. In the case of the sun (no falloff) - that means all the way through every object and right on to the edge of Bryce world (wherever that is).