Thank you for your bug report.
I'm not sure if globally assigning the libphp5.so to any module other than worker.c is the best approach here.
There are a number of threaded MPM available to httpd:
IUS is assuming most users will be using vanilla httpd which provides the two major in the for front (worker and prefork).
The php53u SPEC installs php.conf as a configuration file:
What this means is any changes made outside the RPM (your own configuration) will not be overwritten.
I think possibly the best approach for these 3rd party MPM would be for the installer to then
configure php (php.conf) to use the appropriate PHP library:
LoadModule php5_module modules/libphp5.so