Black linen suit, what shoes?

White dress shirt, black shoes. The t-shirt thing only works if you are very fit and at least a little muscular. Otherwise it just looks too casual. You can only wear black shoes; brown, blue and burgundy don't work with black in my experience.

Personally I would go with dress shoes, but thas because I don't like loafers. I said white shirt mostly because Black Linen is fairly rare, so the interesting part of your suit is going to be the texture. If you are going to have a "feature" item, its better to have one interesting thing among some predictable ones. If you change everything about a normal outfit, it looks like you failed to do it well. If you alter one thing, it looks like you have flair. Just an opinion and as always YMMV

A suit, even a casual suit, looks silly worn with a t-shirt. I'd wear a collared shirt with black shoes.

Can you post pics? While black suits are usually for formal occasions, I'm intrigued by this: the formality of the color being offset by the informality of the fabric. It's the same reason I like black bluchers and suede oxfords.

My choice:

Black oxfords, quarter brogues if you have them - to go with the spirit of mixing formal and informal.

White dress shirt for the exact reasons Blackhood gave. A black linen suit is plenty interesting as it is, I like to have only one "look at me" thing on at a time.

Black grenadine tie.

Again, post pics if you can, or at least details. Very interesting.

Linen suits are the only suits I'd wear loafers with, I think they'd be okay.

If you must wear a T-shirt go round necked rather than v-necked, oh, and avoid polos. A White oxford button down would probably be your best bet.

Since it is a linen suit, loafers would be an acceptable option, but you could also wear black oxfords. Both choices will look great.

Personally I would avoid wearing a T-shirt. If you want to look more casual and relaxed, a much better option
would be an unbuttoned shirt without a tie.

As mentioned above, a white shirt is an almost perfect combination. On the other hand, wearing a black shirt with your black linen suit would be less -as you say- "standard", but the contrast created by a white shirt will make the texture and uniqueness of the linen to stand out more.

This is not the place to go if you want to get advice on t-shirts under a suit jacket. Maybe SW&D could help in that department.

Definitely do loafers. I can imagine a soft leather loafer (think Prada) doing the trick here to keep you from being overly formal.

Don't wear a pocket square with a t-shirt.

+1

I often wear loafers and polo tops with my casual suits.

This party isn't going to be for another few months, right? Linen season won't be here until Memorial Day.

Are you sure that the suit isn't charcoal? Is it really black?