Update pathauto alias without node_load?
Nope, not without circumventing the pathauto module logic (e.g. altering the alias directly in the DB). 

The main function that gets executed in pathauto node updates/inserts, pathauto_create_alias, requires a parameter, $data, passed to it. In a node context, $data is the node object. Without this node object data, pathauto would not be able retrieve data needed for tokens pattens (e.g. [node:title]).