Your limestone rock is "live" rock in the sense that it is a surface that is hosting bacteria. Which is the prime function of "live" rock. Its all about surface area. The reason coral based rock is better is because it has tons of little holes in it, thereby increasing the total surface area available for colonization. Also, different types of bacteria thrive under the various surfaces of the rock. Some prefer the conditions deep inside the rock. e.g. aerobic and anaerobic nitrifying bacteria. Coral based rock is also better in a reef system because it is made up of aragonite which is a less mature form of limestone. Aragonite has the same chemical formula as calcium carbonate but a different geometric arrangement of its elements. Aragonite dissolves at a higher pH which is why it is considered better in a tank than calcium carbonate (limestone). But this is primarily only a concern for reef systems. So long and sort of it is your texas limestone is the perfectly fine for a FOWLR system as it is alive and does serve the structure function. Its just not as "alive" as a aragonite rock. If you wanted to make it better, you could drill small holes in it, increasing total surface area.
Nick