Can an indicator for a latent variable be used elsewhere in the structural part of a structural equations model as an independent variable? I think the answer is no, as I've not seen it done, but would like to check.

Yes, it can. You don't typically see it because usually what you want is the latent variables predicting each other. That's why you're doing SEM. With that being said, it is possible. I've attached a few sketched diagrams of possible SEMs. Of the two circled as "Equal Models", I've seen the upper one diagrammed as a confused researcher thought they had a "factor" with three indicators and the other variable was an "outcome", but it's the same as having a factor with three indicators.

Yes, it can. You don't typically see it because usually what you want is the latent variables predicting each other. That's why you're doing SEM. With that being said, it is possible. I've attached a few sketched diagrams of possible SEMs. Of the two circled as "Equal Models", I've seen the upper one diagrammed as a confused researcher thought they had a "factor" with three indicators and the other variable was an "outcome", but it's the same as having a factor with three indicators.