As to a dysgraphia link, my general take from personal experience and cognitive theories and much reading in the last year runs this way:
When there is a system control failure and the automatic systems are unable to carry out the needed job, the brain might engage in more direct control. That more direct control on the one hand can bring more of the automatic brain systems under direct control and make additional resources available for general cognition. Conversely, when the specific challenged system is in use, the general cognitive resources are tapped out to directly manage that system.
So, with joint mobility issues (hypothesizing that not having typical resistance and range of motion constraints would require much more nuanced muscle control moment to moment) and vision issues you have two vectors demanding cognitive control, which means that there are few available resources to actually think about the coherent construction of what is to be written. Any sort of anxiety would likely also drag down EF resources.
