in my view the first stage of SpaceX Falcon is the thing which is most close to the hypersonic transport today. One can imagine that such reusable stage would accelerate the high-altitude/suborbital plane thus solving the task of having engines for different regimes. Cost-wise - the few millions such a reusable stage launch would cost isn't that far from ~$35K/hour costs of SR71 on missions of 10+ hours.