Powershell Test-Path fails to find registry entry

You could always query the local IP through WMI and store it in $localIP and then match that against whatever address is currently next in your pipeline or array:

if ($localIP -eq $otherIP) { get-wmiobject without -credential }    
else { existing query }

In PowerShell the if statement can be used as an expression:

$i = 'HKLM:\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER'
$SQLPaths = @{
   'DefaultData'= if (Test-Path "$i\MSSQLServer") { 
                     (Get-ItemProperty "$i\MSSQLServer").DefaultLog
                  } else {
                     $null
                  };
   'RepWorking'=(Get-ItemProperty "$i\Replication").WorkingDirectory
}

Also if you're using the PowerShell Community Extensions (PSCX) it adds a Invoke-Ternary (alias ?:) function which could make this less verbose:

$i = 'HKLM:\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER'
$SQLPaths = @{
   'DefaultData'= ?: {Test-Path "$i\MSSQLServer"} `
                     { (Get-ItemProperty "$i\MSSQLServer").DefaultLog } `
                     { $null };
   'RepWorking'=(Get-ItemProperty "$i\Replication").WorkingDirectory
}