So all those "Resource" references I found (and didn't recognize) in the .LAYOUT files pointed to particles. That certainly answers my question.
What will alert you to look into further are all those lines that have
<STRING>DESCRIPTOR:. These are things the .LAYOUT will need to execute properly. Descriptors can be...
<STRING>DESCRIPTOR:Camera Shake
<STRING>DESCRIPTOR:Damage Shape
<STRING>DESCRIPTOR:Layout Link Particle
<STRING>DESCRIPTOR:Timeline
<STRING>DESCRIPTOR:Sound
<STRING>DESCRIPTOR:Unit Spawner
(I think that's it, but there may be others)
The Particles should be easy to find since they are referenced explicitly by their .layout filename with a full path to boot.
Sounds are a little tricky because they are referenced by their GUID. You will have to find that GUID in one of the several .DAT files in the MEDIA\SOUNDS folder (player.dat, skills.dat, etc.) to find out which actual sound files are being used.
I forgot to mention in addition to the Particles and Sounds, the Unit Spawner is important too because it spawns any units (duh) that the skill wants to call.
Within a
<STRING>DESCRIPTOR:Unit Spawner "block", the unit referenced is the one in the line
<STRING>Resource:.