Future-defined CLUT for bitmap visualizer

[Luke-Software1024]

I'm working on making a hexpat for the TIC-80 Cartridge format. In this format, the TILES and SPRITES chunks contain bitmap data, but the PALETTE chunk contains the CLUT. However, PALETTE is

1. at an unpredictable location, and
2. usually after the TILES/SPRITES chunk.

How do I make a CLUT that is only defined later in the file?

[paxcut]

For any type of variable you can use the place operator (@) to set the address where the type will be placed.Let's say that you obtained the address where the LUT is located and that you stored the address in a variable called address which can be any valid address in the file. Further, assume you are at the place where the bitmap containing the indices is located.
Defining:

struct Bitmap {
    u8 bitmap[indexCount];
    u32 lut[255] @ address;
}[[visualize("bitmap",bitmap,lut)]];
Bitmap bm @ $;

or something similar should create the visualization.

[Luke-Software1024]

Typically the bitmap chunks are earlier in the file than the CLUT, so I don't have the CLUT address when the bitmap is defined.

[paxcut]

The visualization doesn't need to be created when the bitmap data is read. Store all the needed parts in variables and wait until you have all the needed parts to create the visualization.
I realize that this is rather vague, but It is a bit difficult to express what you can do in the pattern without some minimal example code that shows where you are having a hard time.

[Luke-Software1024]

I want some minimal example code.

[paxcut]

I am including code to read a paletted bitmap when the lut is found after the bitmap data leaving what is located in between as unwritten code because i don't know the details.

Simplified input file. A paletted bitmap image with the number of rows and columns in image followed by the array pf u8 indices to the lookup table. After the bitmap data there is unspecified data that is followed by an array of 256 32 bit integers representing colors in RGBA8 format. the lut comes after the bitmap data.

struct Bitmap {
    u32 rows;
   u32 columns;
   u8 indices[rows*columns];
   /* unknown data may be here  that needs to be read*/
   u32 lut[256];
} [[visualize("bitmap", indices, rows, columns, lut)]];

Bitmap bm @0;

The format needs to specify how to obtain the location of the lut but in this example i am assuming you can just read the data located in between.