Is there a way to make a 32-bit package variant of mysql-client, for example ? Because I need some libraries for some binary software, and it would be good to maintain them from ports instead of making them manually.
Yes, there are several ways. The most straight foward would probably be to setup a 32bit chroot or jail and build the package in there via make package. Another way is to use one of the many tools in the ports-mgmt category, such as tinderbox or poudriere which are designed for building packages.