I have a few small concerns about these implications, along with one big concern.
Strong no to mechanical_halo. Mechanical_parts is for body parts only. Halos are more of an accessory than something integral to the body.
Mechanical horns is undefined and needs to be cleaned up before it can implicate anything. Many of the posts with this tag are for worn items like headgear (post #2999667, post #3088833) — not body parts — as well as horns like post #3782341 that don't even look "mechanical".
I'd be more inclined to accept a mechanical_ears implication if it were all parts that look completely robotic/mechanical in nature like post #3737836, but this tag is also full of things like post #2891078 that seem indistinguishable from any other fake_animal_ears. Forum topic #14995 did little to clear this mess up, IMO.
Besides the specific problems above, there's a much bigger general issue here: mechanical_parts has a relatively precise definition and a ton of restrictions on its use, and the other tags have very loose definitions or none at all. For instance, tagging post #1405177 and post #2805662 with mechanical_legs makes sense on an intuitive level and it isn't forbidden by the wiki, but neither image is mechanical_parts because that tag isn't for mecha or nonhumanoid robots. Likewise, there's no rule against tagging post #3640332 as mechanical_arm, but it isn't mechanical_parts; this tag is not to be used for parts where the only indication of their mechanical nature is seams along the joints.
I don't think the convenience of having mechanical_parts getting automatically tagged is worth the mess these implications would make of it. This tag was never meant to be an umbrella tag for mechanical bits of every type. Turning it into an umbrella tag would make it useless for its original purpose.
