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

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