Most drugs are safe while breastfeeding. I've found some BMJ studies, but can't get full texts. This page says it goes through in small quantities - what sort of dosage and delivery mechanism are you looking at?
It talks about the quantities getting through to babies being 'negligible'. It sounds like taking the drug and breastfeeding is the best plan.
I've had 3 pregnancies and breastfeed all babies whilst taking pentasa, which I believe is just a different brand (pentasa is mesalazine - am I right in thinking asacol is also mesalazine?). I was always told it was ok, and it always has been.