できた。
import SwiftUI extension ObservedObject { var readOnly: ReadOnlyWrapper { .init(self) } @dynamicMemberLookup struct ReadOnlyWrapper { private let object: ObservedObject<ObjectType> init(_ object: ObservedObject<ObjectType>) { self.object = object } subscript<Subject>(dynamicMember keyPath: KeyPath<ObjectType, Subject>) -> Binding<Subject> { Binding(get: { self.object.wrappedValue[keyPath: keyPath] }, set: { _ in assertionFailure("Read-only") }) } } }