How do you define your functions - if you put them in a separate jar (with faces-config + taglib) it should be ok i think.
The function class is in my ejb-jar with my taglib in WEB-INF. As said, all is working fine for normal page rendering, but not for mail rendering. Doubt it is the location of the class file - I could try and put it under WEB-INF/classes I guess...
Have you tried packaging it as a separate jar (with META-INF/faces-config.xml + META-INF/your.taglib.xml) and packaging that in your .war WEB-INF/lib?