I do not know the 20 bore Lyman "Sabot" slugs' diameter, but the 12 bore 'sabot' slug is about .690" give or take a few thousandths. This is roughly what a "Full Choked" barrel measures at the muzzle - 12 bore, of course.(40 points of choke from .730")
Carrying this train of thought, a 20 bore full choke with 40 points of choke would be .575"- that may or may not be the diameter of the Lyman 'sabot' 20 bore slug.
If you call Lyman's tech. department, they should be able to tell you what the diameter actually is. Due to it's shape, with out a real grease groove, how would you properly lubricate it? At a cost of $95.00, is it worth the experiment? BTW- filling the 'waste' with lube would take a LOT of lube. If it did not sluff off evenly, the accuracy would be effected. The skirt is actually too thin to withstand anything but a squib load, if cast of pure lead. If cast of alloyed lead, it may not expand to seal and would only be useable in a 58 cal. rifle to start with, depending on size of the slug. -
It WILL be undersize for a 20 bore smooth bore and will probably not shoot will in one anyway - without the wad. With a plastic wad to fill in the windage, the BP flame will melt the plastic wad and coat your bore with it.
I've seen reports saying the 12 bore sabot slug shoots reasonably well in smooth bored 12 gauge shotguns of cylinder bore - good enough for deer at that range. I have not 'heard' about the 20 bore sabot slugs from Lyman.