if it's feasable i put a thick, dashed separater line into my composite wall make-up. it doesn't always work well for every type of wall, and can stuff up the dimensioning if it's not thought through, but generally seems to work.
if this method isn't feasable then as matthew said - you're in to simply annotating it after the section has been built . . .
~/archiben
b e n f r o s t
b f [a t ] p l a n b a r c h i t e c t u r e [d o t] n z
archicad | sketchup! | coffeecup