I'm not sure how "hard-coded" it is, but it felt like it at the time given my knowledge of the game files - in that you can't add more buttons to the select screen, nor can you change which party slot corresponds to which button. The thing about Handmaiden and Disciple is, although they take up the same physical button on the select screen, they actually inhabit different positions on the table under-the-hood. This is why I first starting looking at potentially just adding another button in the GUI and letting Handmaiden correspond to that. I couldn't much make heads or tails of the GUI files, and I certainly didn't see anything linking the buttons to the actual party table. So that idea flew out the window.
The next approach was to find a way of making Handmaiden take up any other slot on the screen. Sadly, the only place she'd fit without actually replacing another party member, or potential party member, is right where she already is. Then I considered G0-T0, and having him be replaced as a party member in general so Handmaiden could take up his spot, and just letting him be a floating dummy on the player's ship. Here's the problem with that - work. I would have needed to comb through the game, remove every call specifically to G0-T0 (not associated by object tags) in both dialogs and scripts, and also change every call to the Handmaiden (not associated by object tags) for the purpose of putting her in a different place on the party select screen. And for all that work, I'd actually be removing G0-T0's content (which is tied to influence) from the game.
I looked at this and decided it was unfeasible. So, we have the item, because where she is is actually the best place for her. It was a compromise I made that was the best choice all around.