Hooks In Checkout

Seems that in V4, the checkout:checkout_steps hook is now hidden in step-one when using anonymous checkout or the user is not already logged in. One must go through the billing step and then edit step-one to get a label/checkbox visible.

I looked into adding the checkbox I need to the end of the profiles fields in the billing info (actually the best place for my needs) but they don't use a 'name' for the foreach loop so there's no way to detect when one is at the end (the hook used is within the foreach loop). Could assume that zipcode is last, but this isn't necessarily true in all cases.

So my question is: What hook might people be using to add customer related fields to the checkout process without having to add it to the profiles fields in the admin?

I had also looked at doing it withing the T&C's, but if the site doesn't have T&C's enabled, it won't show there either…

I use the 'notes' hook for other addons, but this one is specific to the customer so not really relevant to put it in the shipping step.