About the variables in custom actions: You can use the following placeholders in the command which calls your asset action from the asset page of a specific asset:
{actionpath}, {smartname}, {assetname}, {dnsname}, {computer}, {domain}, {tag} (which means serial number), {username}, {userdomain}, {ipaddress}, {fqdn}, {scanserver}, {assetid}
{smartname} changes to IP address, FQDN or Assetname based on asset type.
Just an FYI: available parameters are listed in the popup you see when hitting the Add Asset Action or Add User Action button.
Stole from Susan.A and Daniel.B on another forum post.