I believe I’ve figured it out, although it is more of a pain.
For someone trying to do this from scratch, open a retroarch game with the Genesis Plus GX core, and go into the quick menu/options, and find the names of the labels.
Edit: I’ve already done all of the legwork, here are the names of the labels:
System hardware
System region
System lockups
System bootrom
CD System BRAM
68k address error
Cartridge lock-on
Master System FM
YM2612 DAC quantization
YM2612/YM3438 core
Sound output
Audio filter
Low-pass filter %
Blargg NTSC filter
LCD Ghosting filter
Borders
Game Gear extended screen
Core-provide aspect ratio
Interlaced mode 2 output
Show Lightgun crosshair
Invert Mouse Y-axis
CPU speed
Remove per-line sprite limit
For example, System hardware. Open the libretro.c file in notepad++, and do a search for System hardware. (maybe do a ‘count’ also to make sure there’s only one). Once you’re where the label is, you should see something like this:
{ “genesis_plus_gx_system_hw”, "System hardware;
All I did was alter it to this:
{ “genesis_plus_gx_system_hw__gg”, "System hardware;
The reason I chose to use two underscores is, I specifically did a ‘count’ of _gg, and it came up 12 other times. So I wanted to change it to something unique, that way later on, I can do a find and replace all and only affect the altered fields. This will be a huge time saver.
But you’re still not done yet. the “genesis_plus_gx_system_hw” links to another part of the file, and you need to now do a search for genesis_plus_gx_system_hw and find where it links. You should see a string of text that looks like this:
var.key = “genesis_plus_gx_system_hw”;
I simply renamed it to this (to match the other one)
var.key = “genesis_plus_gx_system_hw__gg”;
You will need to do this for all of the option labels, if you don’t want them to conflict with each other on each split core.
The good news is, since I’m having you use a unique edit (the __gg, that we previously made sure did not already exist in the file anywhere), all you will have to do is find and replace all instances of __gg with __sms for Sega Master System for example, when you go to compile the Sega Master System split core.
Again, always double check and make sure there are no instances of __sms before you do it (again, double underscore and not just single).
I’ll try to get this working on my end for all cores. I will not be testing it extensively, as I have better things to do. I’m doing this as a courtesy - if someone else wants to test them for me that would be much appreciated, and if I did manage to screw something up, I’ll fix it, this one time. 
DUDE! That’s so awesome!These boot screens are like the creme of the crop in emulation for me hahaha! I’m gonna try playing with this in the next couple nights, see if I can’t get it running proper in my setup. You’re amazing man! lol