There is no highland equivalent to morning dress, but it appears that some are wearing forms of evening dress for day time functions.
I'm not familiar with "morning dress" myself, but I think the Argyll with a (plain)tie (not a bow tie!) is perfectly suitable for daytime dress occasions.

The PC is pure formal evening wear...black tie...and should not be worn during the day. Certainly not when things are happening in the open air.

For "normal" daytime dress occasions a tweed jacket will do also, but when the military is wearing funny hats, big knives and decorations...the Argyll looks perfectly suitable to me. It's a nice in between solution non kiltwearers don't have.