The 'key' part has to be a property of the HostSystem object, so you would have to look for a property under Extensiondata.
And HostId doesn't exist there.
You can use nested properties for the Key part. So "MoRef.Value" is not accepted either.
Afaik the -Filter parameter only accepts hashtable{string,string} arguments so you can't pass a MoRef in the Value property either.
But why can't you do this with the info you have
$t = New-Object VMware.Vim.ManagedObjectReference
$t.Type = "HostSystem"
$t.Value = "host-139"
Get-View -Id $t