Welcome to the forum. No, unfortunately the AllCast app will only work with proper Roku boxes, the NOW TV boxes have been modified with fewer features for a cheaper selling price. The only option on the NOW TV boxes is the PlayTo app (app from App Store on box with companion app on Android phone) but the reviews aren't great so check yourself before you decide.
NOW are looking for users to join their Customer Feedback Group and help improve the service. If you're interested read more here and sign up.