Great idea.

However, you (or someone) still has to get under the porch to help install them and do the final caulking.

Here is a link that might be useful: Installing Glass Block Windows

when we poured our basement walls we left spaces for glass blocks 24 x 36. We doubled these for security purposes. We like them, the blocks brought in more light than they did when open. They reflect light in the entire basement. Might be a bugger for you to get to the area to replace, but I think you will be pleased.

Here is a link that might be useful: {{gwi:1381758}}

Does your basement have 2 entrances/exits? If not I would want windows that open just in case you're ever down there and a fire breaks out.

Goodness, I live in a basement apt. and I can't imagine not having windows just to be able to look at the sky once in a while. My mom has glass blocks in her house and everyone hates them. They let light in but you still feel claustrophobic. I wouldn't use them in a basement for anything in the world! Also for the right barometric pressure to support your home air is needed to come in and out. Open windows in the summer help keep the basement dryer too. I'm an engineer so from a structural perspective, I wouldn't do it either.

I don't know if you've made a decision on what to do yet, but I had been looking around for similar solutions for my garage. The house is built into a slope with the garage tucked underneath. The one window in the garage is an aluminum slider roughly at outside ground level along one side (near ceiling of the garage), and I think it presents a security problem. I have a curtain on it now so that no one can see in, and there is vegetation blocking the view from outside, but I still think about that when I'm here alone.

I want the light from it but not the security concern. There are other openings to the garage -- a door to the house entry and the garage door itself. Anyway, I was digging around online and found a manufacturer of replacement windows made from acrylic block. They come in fixed and openable forms; I'm attaching a link that I hope works. I'm not in a position to take this project on now, but it's on my back burner.

Assuming that the existing windows were properly framed, what is the structural concern, mariascottage? Particularly for windows under stairs/decks, I can't imagine view is an issue.

Here is a link that might be useful: acrylic block replacement windows