I'm reasonably certain the speed limit (so to speak) on the SR-71 wasn't to prevent its skin from melting.
The hottest the skin got during flight was less than 600 C. That's definitely hot--but it's a long ways short of the melting temperature of titanium (1668 C).
Early supersonic aircraft often had control problems, because the leading edge of the air foil would cause a shock wave that separated the air flow. The control surfaces at the trailing edge little enough air flowing smoothly that they lost authority.
In the X-15, they combated this by building a vertical stabilizer that was basically a V-shape--thin at the front, but much wider at the rear:
This would induce a lot of drag at low speed, but for the x-15, low speed wasn't really a major concern.
The SR-71 took a different approach, using a pair of vertical stabilizers.
The leading edge of each stabilizer produces a V-shaped shock wave. At (approximately) the normal cruising speed, that V comes back from the leading edge of one stabilizer, and hits against somewhere close to the trailing edge of the other. Thus, we still have a solid air flow across the trailing edge (and rudder) of each, and maintain nice directional stability.
For that to work, however, the angle from the leading edge of one stabilizer to the trailing edge of the other has to (approximately) match the angle of the shock-wave formed at the leading edge. Outside of the designed speed range, that no longer happens.
On the SR-71, the vertical stabilizers are angled somewhat. This gives a (still fairly narrow) range of speeds at which the aerodynamics "work", rather than having only one specific speed. Nonetheless, the difference in separation between the top and bottom of the stabilizers isn't very large, so the range of speeds at which it works is fairly narrow. If you try to exceed that range, your stabilizers no longer work, and your control over the aircraft quickly deteriorates.
Having said all that, however, the answer is a clear "yes", if heat were to become a problem, there are materials available that can withstand substantially higher heat than titanium. One obvious example would be the Inconel X that was used as the skin for the X-15 (which flew at a bit over mach 7). Another possibility would be carbon or ceramic tiles, like those used in the Space Shuttles (or some of the other heat shielding it used, such as flexible blankets).
Those have some fairly serious shortcomings so they'd probably be avoided unless absolutely necessary. Inconel X is quite a bit heavier than titanium, and while the ceramic tiles were quite heat resistant, they were fragile mechanically, which led to a lot of maintenance work on the shuttles.