From my research on fostering and adopting, children with special needs have a much harder time getting placed, but the special needs are usually for more challenging (well, depending on your definition) special needs than diabetes.
Before I ever considered pregnancy being a feasible option for us, I always thought foster-adoption would be our path. In fact, now that we are pregnant, we've talked about still making that happen at some point. I like to visit adoptuskids.org and read the profiles, where you can see quite a number that are listed with special needs.
In my discussions with the social services folks locally who run the foster care program, they say many foster families specify preferences, so it is possible. It creates a lot of challenges for them, actually, because it makes it difficult to place kids that need good families, especially teens (who in our area tend to have to leave their school systems and go to other counties for placement).
If you're open to foster/adoption of kids with special needs, you could make a big difference. There may not be as many who have diabetes, but the need for good families willing and able to take in kids with special needs is HUGE.