[docbook-apps] clean and mode-less param switching ideas

Does anyone have any ideas on how I could affect global stylesheet 
params such as body.font.family for different sections of my output?

eg. a book in two parts - first and second parts use separate fonts.

saxon:assign is too risky and unpredictable.

I rather not recreate templates with modes to switch the font.

I'm probably going to fork out my code to use a different param based on 
context.  Just wondering if anyone has any other suggestions.

Thanks !

~Shane