Update: Mission acomplished!
No one replied to my post but I figured it out on my own (not really tho)! Using the most dreaded method of them all, hex editing! A month or two ago I would've been scared away by hex editing but honestly it's not that bad, especially if you have tutorials and any basic coding experience.
I followed a tutorial for hex editing BW2 in-game trades but made a few changes so that it would work in HGSS. If you want to also edit HGSS trades then follow the tutorial I link below but read the changes I made below. Make sure to backup your ROM before following this tutorial in case you make a mistake!
1. First of all, you'll want to open these bulbapedia links in order to easily look up the hex value of the pokémon you want to trade/ be traded and to look up the item you want the traded pokémon to hold.
https://bulbapedia.bulbagarden.net/wiki/List_of_Pokémon_by_index_number_(Generation_IV)
https://bulbapedia.bulbagarden.net/wiki/List_of_items_by_index_number_(Generation_IV)
2. Next, extract narc file A/1/1/2 from your HGSS ROM (the ROM can be edited or non edited, it shouldn't make a difference usually) using NitroExplorer2.
3. Find the file you extracted, by default it will be a plain file with the name 2. Rename the file to whatever you want but make sure to add the extension '.narc'
4. At this point you can follow the BW2 tutorial and everything should work the same. Once you're done hex editing, make sure to save and reinsert the narc file into the same location that you extracted it (A/1/1/2). You don't have to rename the file again it should work fine.
I don't really have experience making tutorials so I thought it would be easier to link to an existing one but if you need any help with this process I'll try to reply to comments and maybe I'll make my own FULL tutorial. I'll be sure to post about any findings I make. One thing I couldn't figure out is how to change gender requirements since the Blackthorn city trade asks for a female pokémon, quite an inconvinience but it shouldn't break anything.