There are no extra contacts to use, which make an ANSI mod impossible. The board uses capacitive sensing, which registers changes in capacitance across the contacts between when the flipper is raised and when it's lowered. This is sensitive/fragile, and I don't think you could reasonably expect to add additional contacts to the PCB and maintain functionality. If you could, then you run into the issue where most of the places you would want to add contacts are just askew of existing contacts. The versatility of the AT stems from the arrangement of barrels and keys which allows some subset to be used. The XT has no extras, and for the most part, no real alternative key configurations that are possible.
You can look into the XTant, which was a replacement barrel plate and PCB that fit into an XT. I know nothing of the current availability; you'd probably need to track down someone with an unused kit they no longer want. But, if you're motivated enough, you could take this as a demonstration and develop your own alternative PCB.
There are some key caps that could be replaced with non-stepped versions, such as the shift keys and (I think) the backspace key. You could replace the two unit keys (like the bottom row modifiers) with wider keys, but it will introduce gaps. If you're interested in a wider enter key and don't mind a change to the layout, you could join the quote and tilde keys into a single 2u key. Or you could just map the tilde into another enter key to make it more reachable.
There is one mod that you could do, which I've never seen done but which should be possible, to change the right column of the number pad to have two keys of 2u height, instead of a minus and a three tall plus key with a single key's worth of contact. That would involve adding two extra holes to the plate, putting barrels through them (with appropriate vertical stabilizer inserts) and using modern style key caps. I don't know if no one's done this because no one's thought of it, or because no one cares enough about the numpad on their XT to bother. But the spacing for the contacts is correct to do this.
More creative solutions could include inserts to replace certain keys with those of a different switch type (like Cherry MX), and hook those into whatever adapter you're using. I've considered trying that for the bottom row, though have not acted on it. Perhaps I should see about giving that a shot. For the XT, getting some more keys in the bottom row would certainly be nice. And this certainly could be a possibility for the enter key, though it would involve removing barrels.
I have other ideas, but they seem to get even less plausible from here, and I've already begun to ramble.
