The SDHI LES has a pitch control motor system that essentially acts like a solid-fueled RCS thruster, designed to help steer a pod during abort. At the moment, it uses the default tiny white plume RCS FX.

Paging @NathanKell to see if custom engine FX are now possible for RCS thrusters.

As of KSP 1.2.1, a variant of the stock ModuleRCS called ModuleRCSFX now supports custom RCS FX.

Note: EFFECTS() nodes generally assume that the direction of the thrust transform is in the Z axis, since they were originally implemented for rocket engines. RCS thrust transforms use the Y axis, so a localRotation = -90, 0, 0 needs to be applied to the MODEL_MULTI_PARTICLE used for RCS FX.

The SDHI LES pitch control motor RCS now has a larger, more visible yellow plume RCS FX, along with the corresponding sounds.

According to @NathanKell, ModuleRCS currently only supports old-style stock FX groups, and defaults to fx_gasJet_white

Setting fxPrefabName = to fx_exhaustFlame_yellow_tiny resulted in a virtually imperceptible exhaust plume, while fx_exhaustFlame_yellow_small and fx_exhaustFlame_yellow resulted in spluttering effects.

I'm keeping the default fx_gasJet_white for the time being, at least until ModuleRCS gets support for the new EFFECTS nodes in future versions of the game.