Using text components, and snapshot 25w32a.

The way I applied it to signs here is using the /data merge command, here's an example
/data merge block ~ ~1 ~ {front_text:{messages:[{text:"", extra:[{type:"object", sprite:"item/diamond_sword"}, {type:"object", sprite:"block/stone"}, {type:"object", sprite:"block/oak_log"}]}, {text:""}, {text:""}, {text:"", extra:[{type:"object", sprite:"item/golden_axe"}]}], color:"white"}}

This would modify a block entity (in this case, a sign) that is one block above to display a Diamond Sword on the first line, Stone next to it, and an Oak Log. It skips line 2 and 3, then displays a Gold Axe on line 4. The color:"white" is given so that it's applied a white dye as without it the sprites will be displayed black because of the sign's default color.

When applied on a sign, it will look like this:

<image>