The Highlanders normal heavies can be feinted, albeit slowly, so use with caution. And of course the Celtic Curse' core mechanic is feinting a top heavy to a side heavy quickly.
Your Offensive Stance attacks however, is where the strong feint game is.
You could for example feint an Offensive Heavy into the Kick, which you can then feint into the Grab. Or, one of my preferred moves, is feint an Offensive Heavy into an Offensive Light. It drains your stamina, and doesn't do a lot of damage, but it's a good mix up, and can keep your opponent from attacking.
try whiffing too. Going backwards with a light attack for example and luring your opponent in to trade with you / getting hit by the follow up blow. You can also always go for a side heavy feint into light/crushing counter which works sometimes but be vary due to the windup is way too long to use it constantly and not being punished for it.
Other than that, as Tundra already stated, in the Offensive Form Mix Up his feint game shines. There you have plenty of options, don't forget that you can always quickly cancel offensive form and go for a guard break or whatnot.
Actually, Highlander is also be able to hard feint the Heavy attack on the Offense Stance into Celtic Curse. It is very useful for people who like parry and dodging.
Usually, when we launch heavy attack from offense stance, they will react to dodge or Parry because it is unblockable, so hard feint into Celtic Curse will hit them hard. In Dominion with Winner Advantage Feats, it is easy to destroy your opponent since it gives hard damage to them.