wedding guest attire

[GG]

I've been invited to a wedding in May and the bride has ask me to wear my kit - but it's going to be a California casual affair and a full formal Prince Charlie isn't called for -I'm a guest rather than a member of the wedding party. Also, the wedding is in California which I expect will be pretty warm. Ergo, an Argyle w/waistcoat is also probably going to be a bit much. Perhaps even a necktie is too much.

Any suggestions on an assemblage of parts to look nice without the kiss of Formality?

Lucky guy.
First thing I would do should be defining the meaning of ‘casual’. You might simply call the bride or groom and ask them their expectections.
Dress tails and tux must be out of question. But could it be a dark business suit or something equivalent?
That would in your case mean a nice woolen kilt, a black kilt jacket (Argyle w/o waistcoat), an appropriate shirt, kilt hose and a semi dress sporran. A neck tie is hardly to expect, but here you can make last minutes adjustments.
More likely ‘casual’ could be pants and a light jacket – in my world. A tweed jacket might be too warm and I should accordingly stick to the Argyle (Argyles are often available in other colors than black).
If over dressed in the jacket just take it off. A plain day wear sporran might be better than the semi-dress. Kilt hose should be mandatory.
If really casual, ie. blue jeans, wear a casual tartan kilt with a day sporran or perhaps even a Utilikilt Mocker or equivalent. Kilt hose rolled up or down. Probably I should still bring the jacket, just in case.

Well, I'm a Dane - different culture. I might be wrong.

Greg