The default BeanResolver that comes with EL looks for JavaBeans Properties, which require void setters.
I imagine it wouldn't be hard to write an ELResolver that would look for non-void setters. If you do, (and it seems like an easy thing to do, though I've never done it), then you might consider submitting a patch, and maybe it'll get incorporated into Seam. :-)
I think there is merit in sticking to Java conventions. Perhaps at some point Java will have proper language-level properties. Until then, we have the JavaBeans conventions.
That being said, we welcome contributions to jboss-el. If someone implements this cleanly, I'm pretty sure we'd let it in.