Warning: If you have never played BG2 there is a pretty major spoiler here. For the ideal gaming experience I would recommend that you back out of this thread now and play the game first, then return here afterward if you're curious about this mod.
At long last, thanks to @CrevsDaak the Gibberlings 3 mod Keeping Yoshimo is now available at Gibberlings 3 for download for BG2EE, both PC and Mac. In a nutshell, this mod
allows players to keep Yoshimo for the entire saga
adds dialogue re: Yoshi's betrayal of the PC (his spying b/c Irenicus' Geas)
adds an entirely new subplot for Yoshi with dialgoues and an encounter
adds a Tree of Life dialogue for Yoshimo
The download link on the page linked above will be updated in a day or two. But it will direct to this thread which provides the latest download and version updates, and links to report bugs and compatibility issues with other mods.
This mod has modest aims. Basically, it allows you to keep Yoshimo to the end of ToB. In order to do that a new story was created, a subplot for Yoshimo if you will. It consists of dialogues about why Yoshimo cannot join you for your excursion to Spellhold. Yoshimo removes himself from the party in Brynlaw and then rejoins the party after you have completed Spellhold. We also decided to add an encounter (battle) just for fun. The mod also includes the option to change Yoshimo's alignment to Chaotic Good.
If you can give this mod a go and provide feedback either at the G3 forum or here if that's easier for you, we will greatly appreciate it!
Major props to @CrevsDaak for taking this on. The mod has languished in Alpha status at G3 for the last five years due to lack of a coder, and Crevs graciously stepped up to update it for EE and Mac. I wish we could give him a boatload of promotes for it, but he clearly deserves at least one!