KDE-3.1-r1 works on sparc, so it should be safe to add ~sparc to the ebuild. My specific install was built against Qt-3.1.1-r1, which is masked due to Bug #12538. Works fine though by adding -no-stl -no-xinerama to that ebuild.
Are you using your sparc's sound and have you had any issues using sound in KDE?
Confirmed that it builds and runs.
Closing