No, this is not scamming the system. This is the type of issue the
DAS is designed to help with. I know what you are going through. I have pretty severe IBS and it causes bladder issues as well. I don't use a DAS, but we have one for our autistic sons. Honestly, though, if I was having a particularly bad flare up of IBS that overlapped with a Disney trip, without my kids, I would absolutely get a DAS. In our experience with the DAS, wait times in excess of 15 minutes are rare, but also in my personal experience with a condition that causes "bathroom issues," you will know when you can get in a line and when you can't, and the DAS is perfect for that. It's open ended return window is perfect because you won't have the pressure to return within a specific small window. I mean, I have been stuck in the bathroom for well over an hour numerous times, and something like a traditional fastpass is useless in that situation.
Also, I know of people with UC/Crohn's and IBS who have DAS for those conditions, so it is definitely something that "qualifies."