I think with cooperation between the caster and the wielder than yes definitely using hold action in combination with bonus action you would be able to make it work as a pseudo buff spell, that being said if you wish to use heat metal as a buff spell casting it on your allies plate armor and having him grapple the opponent would be much more effective since you aren't giving him disadvantage or forcing him to roll a con save. as long as the grapple is taking place the armor must be assumed to be atleast somewhat in contact with your opponent so you both would be dealt the heat metal damage.

I think with cooperation between the caster and the wielder then yes definitely. Using hold action in combination with bonus action you would be able to make it work as a pseudo buff spell.

That being said if you wish to use heat metal as a buff spell casting it on your ally's plate armor and having him grapple the opponent would be much more effective, since you aren't giving him disadvantage or forcing him to roll a con save. As long as the grapple is taking place, the armor must be assumed to be at least somewhat in contact with your opponent so you both would be dealt the heat metal damage.