The summon will spawn with custom status 'Decoy'. On this status I will put an FX overlay like you see stone on Fortify skill when used. In the stats editor, Under certain StatusTypes(eg. Status_Consume) this is 'Status Effect' which is where the ghost texture is going. Also under this is 'Material Types' use 'replacement' instead of overlay.
Now in the resource manager I can find 'RS3_FX_Skills_Soul_Overlay_Ghost_04' which looks like the ghost fx, but it's a Material Resource and we need something like 'RS3_FX_GP_Status_Fortified_01' which is an Effects Resource for it to work.
So now go into the Sparks FX editor. Open 'RS3_FX_GP_Status_Fortified_01' and click on the Overlay Material bar. When the sub menu shows, look where it says 'RS3_FX_OverlayMaterial_Fortified_01' you want to click that and in the menu look for the 'RS3_FX_Skills_Soul_Overlay_Ghost_04'. Also change the overlay type to 'replacement'. Now Save the new FX as a new name you can find, compile effect, add it to a resource, then go back to stats editor and put it in the name of the newly made status.
I actually impressed myself with this. Tested as a shout too, fx works fine- viola now you can become ghosts (atleast as an effect).