The problem is that "can loiter over an area" and "SR71-like" are fundamental contradictions.

The whole point of the SR71 is to be stupidly fast, to the point of being impossible to catch. It could outfly missiles, and the mission was done and over by the point anyone could scramble a proper defense. Have it fly circles over a target and you're negating its one main unique selling point.

Want to loiter over a target? Use an RQ9. Want an impossible-to-stop snapshot camera? Use a satellite. So what's left for the SR71? I guess there are some missions where 1) you don't have air superiority, and 2) the target can hide in time from well-known satellite orbits, but 3) the target can't hide with 15-minute warning from a plane - but are those really common enough to justify the massive investment it'll take?