Around about this time I also started attending a rehabilitation facility here in Montréal or had access to ergo therapist. I explained this problem to them in the solution they came back to me was... keyrings.
What's interesting here is not so much the zipper tab itself, as where I got them… And the best thing is I got about 30 of them for free! Enough for me to address every pair of pants and shorts that I own… And then some…

Obviously, this wasn't quite enough to address my needs. So I got on the phone and called Mountain equipment co-op's online customer service directly. I once again explain the situation and what I was looking for to the agent and I would be more than happy to purchase number of them. He paused for a second and said "don't worry about it, I will send out a bunch classified as a repair".
I've always liked shopping at Mountain equipment co-op anyway, then this level of generosity and customer service I thought was just great. Sometimes all you have to do is ask ;-)
What I particularly like about the zipper pulls is there discrete, durable and little plastic knob at the end makes it reasonably easy to grab. There are a couple different types, the ones they sent me have a little small plastic knob. But the better ones I find is you can actually get a form longer plastic knob to grab onto. Those are the best…
Also if you prefer to purchase some or are having a hard time finding some, I find you can order zipper pulls online at CPGear for about $2C a pair. I find this to be a little bit expensive, so maybe it would be worth shopping around at various online military surplus/tactical stores.
Cost: free!
Pro: much more discreet than a key ring, easy to grab… did I mention I got them for free?
Con: there are various types, and the wrong ones might not address your dexterity needs.