Hello...Is it possible to use an erect image star diagonal (either the
90-degree type or the 45-degree type) on a newtonian reflector...to
get an upright, nonreversd image? I know it'll require an unorthodox
head position to look into it, but I won't mind that. I'm wondering if
it'll work optically. Thanks.

I assume you wish to use your Newt for terrestrial objects? You'd
have to use an image erecting prism in the focuser. I'm not sure what
commercial models are available. In addition, you'd probably have to
move your primary mirror up to allow your eyepieces to come to focus
with the prism in there.

Ritesh