I would suggest you need another syllable in here to make it bounce.
Never mind comply with ISO 71233 Limericks. Part 1: Requirements for meter. (psst, Lin - it's 9 syllables for line 2, although you can get away with 8 if it's got the right "bounce"

)
How about "Looking smart in a purplish cap"? (but you have to pronounce "purplish" as three syllables to make it work)
But I think you've hit the crux of the matter Sir Nige - how can we concentrate on meter when we're distracted by pratty dogs named woof? I think this merits further investigation. In the meantime, I can't wait until it's my turn to start a new one, for which I'll use the line "On some men who were quite hard enough"
Anyway, back to the serious manly business of limericking:
Three dogs named Bark Wuff and Yapp
Looking smart in a purplish cap
Said "This rhyme is quite pants"