I need to correct the term of "hot plug" to "hot swappable" devices.
You need to look into the Gentoo SPARC64 documentation project on the installation process. I believe that there is a loader called SILO which allows multiple boot device entries in a LILO interface. I have not used LILO but I do have knowledge that Gentoo thoroughly documents OpenBoot command entries.
Contacting the FreeBSD, OpenBSD, and Gentoo SPARC64 mailing lists to also inquire about this information will also be of help to you.
That is the only system I am aware has such an interface.
You can have two versions of Gentoo, a version of FreeBSD and a version of OpenBSD on a Sunblade.