I don’t know about the Motion Sensor support as that requires using the I2 commands for linking as the Motion Sensor does not support Peek/Poke. However, defining it as a RemoteLinc should have allowed the device to be used in Actions as it is in the device list. I think the problem with RemoteLinc is it has not made it into the DevClusters file at this Beta level . The Attribute pulldown list is dependent on this. I added my RemoteLinc successfully but could not use it in Actions just as you have already discovered. I queried SHN this morning on the questions. Will post back when I get a response.
EDIT: an updated DevClusters file is being worked on. Maybe next week for availability. That should get you a working RemoteLinc definition. I do not expect to see Motion Sensor device definition with this. Because the Motion Sensor uses standard Insteon Group In commands it should be possible to define the MS as a RemoteLinc which will give you trigger support for the three different Groups the MS uses.
There will be message that the device link database could not be accessed when the RemoteLinc is added but the device will be added to the device list. Probably a good idea to do a SYNCH EZSERVE against the EZSrve device to bring in the MS link definitions that you manually created.
For those who read this without following the whole sequence you can add a real RemoteLinc device and have the link database read when the device is added. You have to put the RemoteLinc in linking mode before the device Add to bring it out of sleep mode.