I thought I could use args to extract the UID from the URL but when I print out all arguments in the block as a test, I get only "node" and "389" so it's not seeing the PathAuto URL but the system URL.
Correct.
To get the uid use node_load to load the node details. If you use print_r() to examine the results, you will see that uid is one of the entries in the returned array. So something like:
$node = node_load(arg(1)); $uid = $node['uid'];
HTH