Hi Annie..
good question, and it got me looking. I found a paper in New Zealand Plant Protection on a study testing seed viability in goat feces. They aren't thistles that i am familiar with, but if you are from new zealand you probably know them...californian thistle, scotch thistle, and variegated thistle. (i wonder if scotch thistle is what we call canadian thistle? for some reason they didn't feel inclined to include the scientific names, grumble)
Anyway it looks pretty promising. Here's from the study:
"Of particular
interest is their efficiency at destroying thistle
species, since goats are known to eat their flower
and seed-heads. The present results suggest that
94% of all viable seeds of Californian thistle
are destroyed by their passage through goats.
Destruction is even more pronounced for Scotch
thistle (96%) and variegated thistle (99.5%), as
shown in Table 1."
I think I'd go for it, especially if most are still in the bud stage. If you were worried about it maybe you could contain them to that paddock for a period after they've grazed the seed heads, to let everything 'run through'?
Anyway, here is the link to the paper for you..
Recovery and viability of seeds ingested by goats
I need to get myself some goats!