If you really feel that the style guide needs drastically updating then it's a wonder that you use RISC OS in the first place!
Personally I feel that the time would be better spent in ensuring that applications are compliant with the existing Style Guide. I have done a lot of work on several legacy commercial applications with this goal.
With regard to future applications, in my opinion style guide compliance is best fostered by encouraging developers to use the Toolbox. Acorn themselves recognised this. Whilst it is possible to achieve a style guide compliant interface without the Toolbox, it seems to me that most people lack the motivation and/or skill to do this.
I would like to single out NetSurf for praise, as an application that adheres to the standard beautifully without using the Toolbox. In fact I believe that some aspects of its GUI would be difficult to implement in a Toolbox application. However, this required a lot of work by experienced developers (and even then there were initially minor anomalies; e.g. behaviour of the Return key in dialogue boxes).