If it's fully standalone in Unbound that should be possible, though I don't know what kind of time frame we'd be looking at.

I haven't kept an eye on it but last I saw it required passing in the https requests from something else like an nginx proxy setup but from the look of those docs they seem to have native support now. The library they mentioned is present on pfSense and is a dependency of Unbound already (the ports option DOH is enabled) so all the backend parts appear to be present, just the GUI/PHP config code would need to be implemented.

The larger problem is that it's going to want to use port 443 which complicates GUI access and makes it trickier to use in practice.