I would really like to add a face plate to a thirteenth century kettle helm. The design seems to make sense, and they had the technology. I just can't find any examples of it. I am wondering how much creative licence I am willing to take.

If you search for experimental helmets of WW 1 you'll find what you're asking about.
Not sure of medieval examples, though.
[ Linked Image ]
[ Linked Image ]

http://manuscriptminiatures.com/codex-justini...s339/3953/
Maybe something like this?

That might be just what I was looking for. Do you know anything about this picture? Date, origin, etc?

Source information is on the linked page. I'm not certain that's a face mask instead of a mail coif. It could also be a cloth hat instead of an iron war-hat. Marginalia in manuscripts are often hybrids, zoomorphs, etc., so another example would be better for substantiation. I think a visor would negate many of the benefits of wearing a kettle hat.

I have seen depictions of cavalry wearing kettle hats, so face protection would be prudent.

What element of the linked marginalia makes you think it represents a horseman? Most of the figures in the same manuscript seem more likely to be urban militia.
http://manuscriptminiatures.com/search/?manuscript=3939

it wasn't in this manuscript, but there are numerous depictions of fully maille armored cavalry with kettle helms. Especially in the Crusader's bible.

Your question wasn't whether cavalry wore kettle hats. Your question was whether kettle hats ever had visors in the 13th century. The one example provided might be more reasonably interpreted as a mail coif with nasal protection worn beneath a kettle hat used by a Bolognese urban militia-man.

I know, I am just justifying why a plate defense might have been used over the face

Well, as noted on your other thread...

Robin Smith wrote:

Sounds to me like you've already decided what you're gonna do, and are looking for support... ...Seems like the wrong way to go about it to me. Start with the sources, then draw a conclusion, not start with a conclusion then go looking for sources.

The images of cavalry in kettle helms do not support your theory of faceplates, they weren't there. The simple reason is that if you wanted a faceplate in the 13th, you wore either a nasal with a full coif, or you wore a sugarloaf or greathelm. With there already being three other options, one would not go about unbalancing a kettle in favour of a faceplate.

The early greathelms were just a faceplate with no brim.
The WWI experimentals weren't used because they were too bulky and very imbalanced.

If you want as much faceguard as possible, pick the sort of kettle that extends more downward than outward, having slits in the brim for eyesight. You can then weld bar-grills to the bottom part of it and cover them in an aventail (also works for the more open kettles) or you can weld/bolt an early bevoir to it (they look like w massive plow/wedge and sits from your chest all the way up to your nose).

The last option is probably your best for a face-plate look, where the first option (bar grills) is a helluva lot easier to fight in (I am assuming you want it for SCA or HMB).