Hi there! Welcome to the Forum.
I have something cheezy that kinda works, but I’m absolutely not proud of the code:
my solution
Excess matrix
#v(1em)
$A + B + C = #h(1em) mat(
4, 3, 2, 1, 0;
2, 1, 0, 1, 6;
4, 2, 1, 1, 0;
5, 2, 6, 0, 1;
1, 4, 2, 0, 1
)
#place(dx: -6.3em, dy: -1.68em, $vec(delim: #none, 5, 8, 5, 8, 5)$)
#place(dx: -5em, dy: -3.4em, $ mat(delim: #none, 0, 0, 0, 0, 0)$)
#[
#set math.mat(column-gap: 0.3em)
#place(dx: -3.8em, dy: 2.7em, $mat(delim: #none, "", "", T, T, T)$)
]
$
result
So apparently you can create matrices without the brackets, so I created separate matrices/vectors for the overflow elements and positioned them using the place function. The positioning is relative to the position of the matrix (therefore I put everything in a math block), so it should be possible to add content around it (like the equation in my example). The letter T seems to be wider than the numbers, so I shrinked the column-gap down to 0.3em. The “#[…” is needed for the set “#set math.mat(column-gap: 0.3em)” to only affect the following line (see here). Alternatively, you could set the gap back to the default of 0% + 0.2em below the matrix entry.
If you’d like me to, I could try to create a custom function for it so you can input other values, although the alignment will most likely not always fit this well, especially when using letters, although you could use mono-space letters for better generalization.
I think it might be worth it to open a new Issue on Github for a native support of this. I think it would be a Compiler Feature Request.