What are the safety and warranty implications of steaming bread in a domestic oven?

I think you are being too conservative. The oven door and glass in there is built to handle some temperature differences. When someone opens an oven door and sets a room temperature pan or cooking container on it while moving racks or other food around, I can't think it would be THAT different that water splashing around. If this were an issue, we'd probably hear about it, a lot.

Pretty much nothing you put into the over starts out as completely dry. Steam gets released from food all the time. Ovens are built to handle steam.

I've used water baths for stuff like cheesecakes. It's very common and if an oven failed because of this, it would be a defect in the oven, and covered under warranty.

Turns out my self-cleaning gas oven is designed to have water in the bottom so as to use the steam generated to clean the oven. I cannot think of why this would cause a problem in your oven. The electrical elements are very protected from water or steam, no worries there. The meaningful electronics are all in the top back where you hit the buttons, completely isolated from the oven. Nope. Can't think of a single reason why you shouldn't be A-OK with adding water. Personally, I'm glad you put the idea into my head as I foresee some steamy bread baking in my future.

Regarding the safety of the door specifically:

Oven door glass is made of tempered glass. This does have a maximum temperature differential that is near reasonable cooking temperatures, so it is possible to exceed. This can cause the glass to shatter and/or explode impressively. That being said, tempered glass shatters into less dangerous pieces than regular glass. (Which is one reason it's used so commonly.) The worst injuries I'd expect would be burns and small cuts where the pieces hit exposed skin. Not great, but not life-threatening.

If you are worried about safety here, my suggestion would be to stick to almost-boiling water instead of cold water when adding water to a preheated oven. That would basically be 200 F water instead of 40 F water, which gives you 160 F fewer degrees of difference between the water and the oven door.

(The things you actually need to watch out for are self-clean oven cycles (these will cause marginal oven door glass to fail) and pushing pans or racks into the oven by closing the door. If they touch the glass, that could scratch the glass, which weakens the glass over time.)