I don't have IBS but I occasionally have problems if my diet changes so I don't have breakfast before I go because that seems to speed things up. I also add Benefibre to my morning protein shake (prepared and drunk at work soon after I get to my cube) which seems to help too.

On a side note, I was thinking about bike touring last night when I remembered that, for the last year or two, I get up in the middle of the night for a P, I need to consider that when I find a place to setup camp.