How can somebody with Type I diabetes keep blood sugar levels acceptable during and after a multi-hour ride?
I too am a type one diabetic and I understand your concerns, how ever I might have some suggestions to help with your problem. I always make sure to carry a Camelbak pack with me to enure I have my Glucometer and other supplies along. I check my sugars before I start the ride, if possible I might have a little extra food at the time, just to get me started. 

I also make sure to carry along some chocolate bars if my sugars get low. I find Oh Henry or Mars Bars to be good, since they have various types of sugar in them and they also have peanuts to help sustain the glucose intake. In addition to the sugar I bring along, I also bring protein bars since those have carbs in them, but also a lot of protein to help release the glucose in to your system slowly. 

The name brand or protein bars I use is called "Clif Builders" and you can get them at Costco in Canada, hopefully you could get them in your area as well or a similar product, the important thing is that the protein bars release the carbs slowly. 

I also drop each of my insulin dosages by 2 units on my morning dose and sometimes my evening dose also gets adjusted because after a long ride your body continues to burn sugar at an accelerated rate. 

This helps me not need to overeat, the high problem of high sugars after a ride is due to over compensating for the amount you fuel being burnt on the ride and most important of all at least until you get to know you routes really well and what you truly need for food intake, make sure to check your blood sugars on the ride before you eat and on a regular basis and bring along extra protein and chocolate bars, they don't take up much space and the Camelbak will have lots of room for them. Remember the Chocolate bars are only for when your sugars get low, otherwise stick to the protein bars. 

You will also find your rides will be better and more energetic if your sugars are in the nominal range. The other advantage to lowering your dosage on riding days, is that it will help you lean down. It doesn't help you at all if you have to overeat to keep your sugars at a normal level, but if you lower your dosage and understand that the bike riding it self will lower your sugars to a degree it will make it easier, it really is about knowing your body how it burns sugar and how to balance it all on days you ride.

P.S. My longest ride so far was 55Kms and I found by dropping my morning dosage, it helped me out in not battling the insulin to keep my sugars up at a normal level. Perhaps if you are going on a longer ride you might need to lower your dosage another unit or two, don't make huge changes in dosages until you know your insulin needs. 2 units less is quite safe and make sure even if you lower your dosage to bring plenty of food along.

I hope you find this information helpful, enjoy the rides.