Will it be backlighted? Could you post some photos?

I think that’s a great idea. We have a source in a nearby town that has what seems like a million different stained glass pieces to choose from. May use your idea for the guest bdrm. Thanks!

I think it's a great idea!

Do it! Just make sure it is SECURE. You don’t want heavy glass landing on your head. Also, keep in mind the true beauty of stained glass is observed when it is lit from behind, so perhaps install some remote LEDs.

Someone just did this I think.

https://www.gardenweb.com/discussions/5590161/cpartists-finished-house#n=92

I was actually planning to install it in the wall during our reno, so it will be backlit from the sun. The current plan is regular windows on either side of the bed & then a transom stained glass over the headboard. This wall is the only obvious spot for a bed in this room, so I feel confident a bed will always be under it

It’s really more of a window treatment (or lack there of) and proportion decision. Typically, you wouldn’t want to cover that type of a window with any coverings so are you ok with early morning sun streaming in that window? Privacy shouldn’t be a concern? Typically the transom would be high up (your bed obviously low down) - but what about the headboard? If it’s lower as well and that creates a big void below the new window then I question just the overall look. You’d have to sketch it all out to scale to see if the proportions work.