I think I got it... both tools just tell the LS service (using port 9524) to start the scan, and the actual WMI queries are always performed from the LS server.
The thing is that apparently lstrigger only works if the machine is already on the database (either discovered by active scan or added by LsClient). If the machine is not there already, the service does nothing.
I was having issues with this and some production servers that are not part of the domain, just using individual workgroups. The funny thing is that I was able to manually insert those machines on the database and they started to show up on the web console and then I was able to start using LsTrigger on them.
I would be nice if LsTrigger would add the machines to the DB, or the service would try to reach the machine even if it's not on the DB, and add it if it's reachable.
If anyone else want to do that, it's pretty simple. Just need to add the machine on the "tblComputers" table (most of the fields can be null and later the service will fill them up). I used the same machine name as the domain name there (since the workgroups on each one are named the same). Not sure if that's supposed to be the machine name or workgroup name, but that's easy to check.