How to implement very basic custom QSys component

You could post a .zip of your _hw.tcl and .vhd file together and someone might check it.

But assuming there isn't some basic error like incorrect connections, "write never happens" could simply be because the software never did it. e.g. writing to the wrong address, or optimizer removing the statements in question. Similar to the use of u32, another easy change is to use the IOWR() macro from <io.h> instead of your straight pointer handling.