Sorry Gerard, I thought you were implying that it was impossible to enter non-keyboard characters directly into the on-canvas text tool.
I do of course know about the character selector applications and using the center mouse button to paste. Amazing that many people do not know that their scroll wheel 'clicks'.
Probably the same as Gentoo, PCLOS (also a rolling release now on KDE 4.14.3) center-button does not paste in Gimp.
but..ctrl-v still works much the same as any other application
Another option for you to explore, for frequently used (certain) characters, is the compose key. Especially useful for currency / accented characters / things like a degree sign.
http://en.wikipedia.org/wiki/Compose_keyexample.