OK so for anyone else who may come across a problem like this, I've finally got the solution. After hours and hours of testing and trial and error here's what you'll need to do to execute a powershell script from a dvrmstoolbox externalrun command.
1. You need to run set-executionpolicy remotesigned on BOTH the 32 bit powershell.exe AND the 64 bit powershell(x86).exe to run scripts you downloaded from the internet (namely wtv-metadatarenamer)
2. You'll need to set the program to run to c:\windows\system32\powershell\1.0\powershell.exe (the whole path is needed, yours might be different)
3. You MUST use this syntax in the arguments window exactly as it is here:
-command ""&" 'c:\< Your Path Here >\wtv-metarenamer.ps1'"
Replace the path with your full script path.
After that it works like a charm, spent many hours trying to figure out the syntax that would pass the parameters correctly.
Hope this saves someone else from a lot of headache.
BTW- the wtv-metadatarenamer polls http://thetvdb.com/ and gets season and episode info, so it's quite handy to use.