Sure, but you need justification for such things, don't you? You shouldn't just put it under Contacts because that's your character's best skill. It needs to make sense. Thematically, Presence makes the most sense, then Conviction. Maybe Discipline, but that's stretching it for me.
Arms haki could be Fists, Guns, Weapons, or Might. Observation haki could be Alertness or Investigation. Supreme king haki could be Intimidation.
So there's a fair bit of wiggle room. At least if you're not using multiple types of haki.
I assumed additional mental stress boxes were not mentioned because the custom power preceeded Mental Toughness from the Paranet Papers. Whoever created it probably only specified the additional physical boxes because only those existed at the time.
I'm pretty sure it was intended to work with
Resilience, but not with
Stoicism. Since PP Mental Toughness is designed for spellcasting, it ought to be allowed with Supernatural Martial Arts too.
We might be able to ask the original author, but I don't remember who it was. Might even have been me. It was years ago.
Anyway, it can work and I'm warming up to the idea, although I still find it more "canon" to have Haki linked to Presence. With SMA, though, it's sort-of linked to Conviction (indirectly) which is the next best thing, in my mind. Any fight against a Logia user is going to leave you in terrible shape, but I guess that's staying true to the manga.
I'm thinking that the hit-Logias part could be free, along with the tanking-attacks part. Haki could be four Powers: a new one for basic arms haki, Supernatural Sense for basic observation haki, Supernatural Martial Arts to expand both, and a new one for the haki of the supreme king.
Speaking of supreme king haki, it's probably just a weak mental attack with an infinite number of targets. Maybe without rolling, since it seems very binary: it works or it doesn't.
Spitballing here:
HAKI OF THE SUPREME KING [-2]
Description: Whatever
Skills Affected: Presence, Conviction, Intimidation
Effects:Something. As an action, you can make a mental attack against any number of targets within your line of sight. Use your skill - 3, they defend with their Conviction, Presence, or Discipline. Everyone rolls 0 automatically. Weapon rating is your skill. You can only target people once per scene.
Something else. You can maybe defend people from this Power somehow, since the Marineford battle didn't have all the mooks falling unconscious as I remember it.
How would you handle this?
Spectral Strike (2 stress): The enhanced attack interacts with spirits and other immaterial things as though they were solid.
Does this completely nullify the Physical Immunity of Logia Fruits?
I'd say no. It obviously wasn't written with One Piece in mind, and I don't think it should be that easy to beat down Logia users.
That's just me, though.