There's no way to do it from TI-BASIC only. I'm pretty sure the border is a relic of the TI-84+CSE. The calculator had barely any RAM and you could not fit an entire screen into RAM at once, so shrinking it by creating a border made it easier to work with. The TI-84+CE has 24-bit memory addressing so it has way more RAM, so this is no longer a problem.
If you want to draw something outside of the border, you're going to need Assembly. If you want it to be compatible with TI-BASIC code, there's no simple hex code for it.
To draw directly to the LCD display, you need to modify vRAM. vRAM is a location in RAM located between memory address 0D40000h and 0D65800h. If you subtract these two numbers, you get exactly 153600. Why? 320*240*2 = 153600. Each pixel on the screen is mapped to two bytes in vRAM. The two bytes represent the color code for that pixel.
This means to modify a single pixel on the LCD display, you need to first determine a 2-byte color code for that pixel, calculate where you want to put it in vRAM, then modify the vRAM at that address.
The color codes are pretty simple. On your computer, you're probably used to RGB. This is how RGB color codes on your computer relate to color codes on the calculator:
f(r,g,b) = 211⌊r÷8⌋ + 25⌊g÷4⌋ + ⌊b÷8⌋
To calculate where in RAM to write the color code, use this:
f(x,y) = 2(320y + x) + vRAM
Where vRAM = 13893632.
So basically, you need to take your color code, calculate the location in memory to modify using the equation above, then write the data.
Here's an example of doing it in Assembly:
ld de, 0 ; clear out deu
ld de, $FBBF ; store pink color code in de
ld hl, 2*(320*200 + 200) + vRAM ; store screen location at (200,200)
ld (hl), de ; write it to the screen
That will draw a pink pixel at (200,200).
If all this sounds too complicated to you… I wrote up a hex code you can use to accomplish this. I tried to make it as compact as possible, but it's still quite long.
AsmCE84Prgm
CD840302
21F905D0
3643
CD600F02
CD700F02
3E10
CD480300
EB
D5
CD840302
21F905D0
3659
CD600F02
CD700F02
3EA0
CD480300
29
E5
CD840302
21F905D0
3658
CD600F02
CD700F02
E1
19
29
110000D4
19
D1
ED1F
C9
Put this code in a program called "PIXEL". You can draw a pixel to the screen in your TI-BASIC code like this.
50->X
50->Y
3071->C
AsmPrgm(prgmPIXEL
X is the X coordinate, Y is the Y coordinate, C is the color code.
Also, for this particular program, to convert RGB values from the computer to the equivalent, use this function instead of the one listed above:
f(r,g,b) = (211⌊r÷8⌋ + 25⌊g÷4⌋ + ⌊b÷8⌋)÷16
Note that having to load a program every time to draw a single pixel is pretty slow.
I might get around to coming up with a faster way to do this through TI-BASIC in the future.