Ah. No. I don't think you don't need to worry about all this nonsense anymore.
There is no incompatibility anymore between Plesk and the ART php/mysql under Centos 6 that I'm aware of. Take a look at viewtopic.php?t=5482
There was a problem, which got triggered when navigating to the APS pages for some reason, with an older version of mysqlclient16 which was missing strmov(). A version with this function was only quite recently added to the Centos 6 repo, and resolves the problem completely.
The Parallels support people do have a point about supporting third party repos, of course. It adds complexity and unknowns, left right and centre. They should think about supporting the ART repos, however, as they are as Plesk specific as you can get, in many ways. To be honest, my opinion they are trying to support too many flavours of linux, and would do better to concentrate on the core server flavours, and then add support for certain third party repos like the ART one. The end result would be fewer problems, imho.
Do be aware, however, that if you upgrade to the ART php and ART mysql 5.5, going back to the stock os-supplied flavour may be painful. Not impossible, but painful. I personally consider it a one-way move. After careful consideration, and assurances from Scott that he'll provide long-term support for 5.3 (rather then dropping it completely in favour of 5.4), I'm going the ART route with all our 10.4.x installs, starting this weekend.
And just to amplify on this, I've been testing with Centos 6 (x86_64), php 5.3.10 and mysql 5.5 from the ART repo, with php-zendguard-loader, php-ioncube-loader and php-eaccelerator and have not experienced any problems with the panel at all. Of course the panel's php is separate, but it does use the common mysqld, hence the problems you were having initially (without the updated mysqlclient16)