Seems like yes this can be done, I'm doing it in PHP on download, simply echo your additional data along with your file and that data can be read by the NSIS installer without recompiling your application without corrupting the binary
echo "mydata:hello,world" >> installer.exe would work too (on any OS) add the function included on the wiki to your nsis script then use something like to call the function and read the data:
Push "mydata:" Call ReadCustomerData Pop $R1 StrCmp $R1 "" 0 +3 MessageBox MB_OK "No data found" Abort MessageBox MB_OK "Customer data: '$R1'"