Map select ROM, choose Option from the pause menu, use A/B to choose a number and press start to warp, you might get asked to nickname pokemon but just skip that.
IDK what IDs work and what ones crash, dunno if all maps are even accessible this way neither, meh
https://anonfilescom/Xeq4japfo1/yellmons_gbThis uses startmap_set & cont1 functions (SpecialWarpIn / SpecialEnterMap in pret), dunno if thats the best method or not.