Once you figure out where the value in RAM is, you'll want to take that value from RAM and display it as a BG tile, so you're going to want to know where the BG map data is stored and how to change it. BG map data can be placed at different places in RAM, and you can use no$gba's Vram Viewer to see the BG maps 0, 1, 2, and 3. It will show you the actual map address, like 6001036 for example. Addresses for the GBA that start with 6xxxxxx are VRAM addresses. Memory that's used to display data on the screen. If the variable that you want to display is from 02038E7C, memory at 2xxxxxx is WRAM (Work RAM) also general purpose RAM. So you'd need to read data from 02038E7C and copy it to 6001036 for example. These are probably not the exact memory addresses but that's for you to figure out.
Using the example assembly code from [Unknown] (because my gba assembly is rusty), your code might look something like this
ldr r1,=2038E7Ch ;load address of variable into r1
ldr r0,[r1] ;load what's at address r1, put it into r0
ldr r1,=6001036 ;load address of map spot for new tile into r1
str r0,[r1] ;store value from r0 to address in r1
It looks like you can use xkas-plus v14+1
to assemble the commands and insert into the ROM pretty easy. Locations starting with 8xxxxxx are ROM cartridge locations and that's where your hack will be stored.
You'll also need to insert a jump into the game's code. This will interrupt the code, make it execute your code, and then return to the game code.