Can you use glass dildos in a strap on?

It can be done, but it's not recommended for a variety of reasons.

Cocks for strap-on fucking benefit from a touch of give and flexibility since they're going to be subject to vigorous movement and you want them to conform somewhat to the curve of the orifice. A bit of give is especially necessary for long cocks as they can bump the cervix and a hard bump can be quite painful. Glass has no flexibility or give, so if you do use it in a harness you will need to be very gentle and careful, and the number of positions you can achieve is very limited.

Overall, I love glass but it's far better suited for handheld play. For harness use, silicone is king.

You can but you should be very cautious.

if the base is flared enough, you can

I won't let strapped-on glass toys in my butt. Yeah, glass is fun, but it's unforgiving and I don't want to have any accidents that leave me bruised or my rectum perforated. I also like toys where I know my wife can really go all out if she wants - I want her to not have to worry about hurting me. Silicone only for strapon use here.