I used to do tons with LD_PRELOAD

Nowadays, if you want to do something like that and also explore exciting underused parts of ld.so, the thing to try is LD_AUDIT libraries. These are loaded in a new dlmopen namespace, so they can rely on other shared libraries and the like, making it a lot easier to write the things than it is to write an LD_PRELOADed profiler.

(In practice this excellent feature is rarely used, alas.