Is there a possibility to update global variables in Canvas so that these can be referenced (and updated) between elements?
At the moment, this isn't really possible. Part of the reason is that there's no guaranteed order of evaluation for each element at the moment. If some elements were changing variables and were being executed at different times, you could get different results each time you loaded up your workpad.
You can however use var_set to override the variable within the element's scope.
Hope that helps!