Volt Absorb: Another utility could be that we can force out some choice locked Pokemon like Koko or Plasmanta, but I don't think it's enough...
Rock Head: If Rock coverage will be allowed, this could be a useful ability, since it grants a safe Head Smash which if powerful than Stone Edge. Also, even Brave Bird benefits of this, but this move alone is not enough
Skill Link: This ability grants all hits on Rock Blast (if allowed) and Icicle Spear. Same comment of Rock Head
Rest: I trust Deck's judgement in saying that CAP23's bulk won't be too much with Early Bird+Rest, but I would like some calcs to show that our checks can reliably 2HKO it, which is what is required for CAP23 not to completely rest stall us to death. Other abilities such as Pressure can also work with Rest sets, but that really hasn't been discussed too much, so some more discussion on this would be great if we still want this to be a possible route we can go.
*NOTE: most of these calcs are the weakest cases, normally a lot of these have a Life Orb or a Z-Crystal that grants the OHKO*
252 Atk Kitsunoh Shadow Strike vs. 4 HP / 0 Def Garchomp: 296-350 (95.4 - 112.9%) -- 75% chance to OHKO
252 Atk Guts Naviathan Iron Head vs. 4 HP / 0 Def Garchomp: 231-273 (74.5 - 88%) -- guaranteed 2HKO
252+ Atk Huge Power Mawile-Mega Play Rough vs. 4 HP / 0 Def Garchomp: 734-866 (236.7 - 279.3%) -- guaranteed OHKO
252+ Atk Bisharp Knock Off vs. 4 HP / 0 Def Garchomp: 302-356 (97.4 - 114.8%) -- 81.3% chance to OHKO
252 SpA Life Orb Syclant Ice Beam vs. 4 HP / 0 SpD Garchomp: 320-377 (103.2 - 121.6%) -- guaranteed OHKO
252 SpA Life Orb Teravolt Kyurem-Black Ice Beam vs. 4 HP / 0 SpD Garchomp: 330-393 (106.4 - 126.7%) -- guaranteed OHKO
8 SpA Ninetales-Alola Freeze-Dry vs. 4 HP / 0 SpD Garchomp: 116-140 (37.4 - 45.1%) -- 66.8% chance to 2HKO after Stealth Rock and hail damage
252 Atk Tough Claws Charizard-Mega-X Dragon Claw vs. 4 HP / 0 Def Garchomp: 452-534 (145.8 - 172.2%) -- guaranteed OHKO
252 SpA Latios Draco Meteor vs. 4 HP / 0 SpD Garchomp: 422-498 (136.1 - 160.6%) -- guaranteed OHKO
252 Atk Garchomp Dragon Claw vs. 4 HP / 0 Def Garchomp: 348-410 (112.2 - 132.2%) -- guaranteed OHKO
252+ Atk Mimikyu Shadow Claw vs. 4 HP / 0 Def Garchomp: 260-308 (83.8 - 99.3%) -- guaranteed 2HKO
252 SpA Gengar Shadow Ball vs. 4 HP / 0 SpD Garchomp: 240-284 (77.4 - 91.6%) -- guaranteed 2HKO
252 SpA Choice Specs Greninja-Ash Dark Pulse vs. 4 HP / 0 SpD Garchomp: 408-480 (131.6 - 154.8%) -- guaranteed OHKO
252 SpA Life Orb Protean Greninja Ice Beam vs. 4 HP / 0 SpD Garchomp: 299-354 (96.4 - 114.1%) -- 81.3% chance to OHKO
252+ Atk Guts Colossoil Knock Off vs. 4 HP / 0 Def Garchomp: 444-524 (143.2 - 169%) -- guaranteed OHKO
0 Atk Sableye-Mega Knock Off vs. 4 HP / 0 Def Garchomp: 164-194 (52.9 - 62.5%) -- guaranteed 2HKO
252+ Atk Strong Jaw Sharpedo-Mega Crunch vs. 4 HP / 0 Def Garchomp: 602-710 (194.1 - 229%) -- guaranteed OHKO
252 Atk Gyarados Crunch vs. 4 HP / 0 Def Garchomp: 226-266 (72.9 - 85.8%) -- guaranteed 2HKO
252 SpA Tapu Lele (Choice Scarf) Moonblast vs. 4 HP / 0 SpD Garchomp: 288-338 (92.9 - 109%) -- 50% chance to OHKO
252 SpA Kerfluffle Moonblast vs. 4 HP / 0 SpD Garchomp: 260-308 (83.8 - 99.3%) -- guaranteed 2HKO
252 SpA Diancie-Mega Moonblast vs. 4 HP / 0 SpD Garchomp: 332-392 (107 - 126.4%) -- guaranteed OHKO
252 SpA Pixilate Gardevoir-Mega Hyper Voice vs. 4 HP / 0 SpD Garchomp: 386-456 (124.5 - 147%) -- guaranteed OHKO
0 SpA Clefable Moonblast vs. 4 HP / 0 SpD Garchomp: 180-212 (58 - 68.3%) -- guaranteed 2HKO