[R] How to write (complex) coercion methods or "what's the reason to limit 'setAs()' the way it is limited"?

[Janko Thyson]

Dear list,

I'd like to write coercion methods for some of my Reference Classes. 
However, using 'setAs()' is not a real option as its argument 'def' only 
allows for functions depending on one single argument. In some cases, 
that is simply too much of a limitation for me. And I don't really see 
why it needs to be this way, so I guess this question is also some sort 
of a feature request to make 'setAs' a bit more flexible (in case anyone 
from the Core Team is listening ;-))

A reproducible example can be found here: 
http://stackoverflow.com/questions/8346654/how-to-write-coercion-methods

Thanks a lot for any replies!
Janko