I'd have it before...as you said, clear liquids for a time followed by a gallon of that prep would be pretty hard.
I have several close relatives with colon cancer, and have had probably 4 colonoscopies by the time I was 50. The prep is the worst part, and I have a normally functioning gastric system. I can't imagine it once your surgery is complete.
Get it done and over, and you won't have to worry for another 5-10 years, depending on what they find and your doctors recommendations. By then you'll be well healed from your surgery and will have adjusted to your stomach capacity issues.
I forgot to add that my father, age78, died of adenocarcinoma of the colon with metastasis to the liver, 2 1/2 years after the colon surgery.
There are no strict guidelines regarding the time of a colonoscopy in relation to the upcoming bariatric surgery. The decision of whether to do it before and after will be dependent on your individual situation, and can be discussed with your personal physician.
As you know, a colonoscopy is a day procedure, and should not have an effect on the upcoming surgery. You are correct that the post-op course of bariatric surgery may be daunting, and may put off the colonoscopy.
That being said, it is difficult to make a concrete recommendation - there will be doctors who will do it either way. There are pros and cons to each side - and should be discussed with your personal physician or surgeon.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
http://www.straightfromthedoc.com