Interesting! Sorry for the late reply here!
I'm not completely following exactly what is going on. Did you change the RunAs user for the content to the userid? In case it is helpful, this Shiny app may be helpful for debugging the permissions and items available to the RunAs user for the content:
That said, it is definitely not something you should expose to anyone more than an admin, as it allows arbitrary shell access as the RunAs user.
Does NAS require authentication via Kerberos or something of that nature? If so, there will definitely be other hurdles to jump through in order to set up a keytab or credential cache for accessing the NAS as the RunAs user for content.
I'm curious to hear what else you have tried, and whether any of these ideas are helpful! A bit more clarity and answers to the questions above would definitely help me understand what you are trying to do a bit better.