Well apache 2.4.1 and PHP 5.4 arent compatible yet so you might want to hold off there. If you want to push it though, I have httpd 2.4.1 packages in the atomic-testing channel.
Other things to be aware with httpd 2.4.1, all DSO modules (perl, python, php, ruby, fcgi, etc) will need to be rebuilt against the newer API. Pretty much if its not a module from the httpd-2.4.1 src.rpm, youve got to rebuild it.
Provided you're packaging everything correctly, plesk is going to work with these at a base level (IE it will detect them, it will display, allow you to stop/start, etc). The only tricky parts, and I think these are when the newer apache or php package has deprecated a configuration token that causes a hard error that prevents the daemon from starting/DSO from loading. To fix that you just need to tie into the plesk API to make a correction after the fact.
Replacing mysql completely might not be an option, at least for the psa & horde databases. You could certainly limit plesk's interaction to just that daemon, and then install paragon or mariadb in parallel for your userland components. Again, tying into the API here is a workaround if there is a major change between the two (creating users, databases, etc)
In the interim, we do have php 5.3.10, and httpd 2.2.22 packages in the repo. Those are both completely compatible with plesk 8.6 and up.