It was indeed originally designed with a drum rear brake - which I'm sure was perfectly adequate for the machine's meagre performance. When they changed to a disc rear they also introduced a new box-section swing arm, but the way the brake is attached to it does indeed seem like a bodge - it's fine until you have to remove the bloody thing and then try to reassemble everything - I spent hours today trying to get the brake caliper to line up with the disc and had to give up in the end so the bike will be off the road for at least another week now. 
I've no practical knowledge of your Trials bikes rear tire R&R procedure. BUT Interceptors suffer from the same problem: getting the rear wheel back in place can be a real ass-ache. The solution on the Interceptor is to remove the brake pads from the caliper, install the wheel, then install the brake pads. Doing it that way turns it into a five-minute job, instead of a five-hour battle. My Interceptor also has a split master link, so I break the chain, rather than wrestle with it while trying to wrangle the wheel in place. If you're not breaking the chain when you install the wheel, you might consider that as well. Long ago I learned it takes less time to remove whatever is in your way than it does to try to work around it.
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