Hand contructure - Dupuytrens contraction
Once I had a client who had Dupuytrens contracture. Here all the hand and the fingers are not able to open up any more, due to extreme fascia shortening.
I worked with heating compressions, massaged with warm medicated Sesame oil, I taught ROM (range of motion) of the wrist and arm and applied some point work to open adhesions.
By letting the hand rest on the massage table the client leaned, while standing, into stretches slowly and applying tension onto the table according to capacity, we increased the range of motion gradually over some weeks.... We did further active/passive/tension/relax exercises increasing mobility. I told to continue these exercises daily and come in weekly. The flexibility increased and later he told me that he cancelled his operation.
I believe in the kind of homework a client can do. The trick is not to overwhelm them. Many of my clients have actually an aversion against movement. Hence, less can be more in this case.
This sounds like an excellent treatment protocol to me. I just finished a major update to my Thai foot massage course which includes a lot of material about a similar issue as you described for the hands: the shortening of fascia or tendons in the feet and calf muscle, primarily because of high heels, and how to reverse it.
As you said, this is a gradual process and takes time. In the case of the feet a lifestyle change can be very helpful (like not wearing high heels). However the cause for this in the hands is not so obvious.