I may get a chance to reply to more of this later, but for now:

Sven Wischnowsky writes:
 > 
 > > It may be useful to interpret return code from such a function (a lá -t).
 > 
 > Agreed. I was already searching for things we could do with the return 
 > value, but hadn't thought about `-t'. Any suggestions how the return
 > value of completion widgets and zle widgets could be used (when not
 > called from `-K' for the former)?

As long as we're revisiting this discussion and adding things like the
new NUMERIC parameter, allow me to direct your attention to these old
zsh-workers postings and any associated threads:

http://www.zsh.org/cgi-bin/mla/redirect?WORKERNUMBER=4067
http://www.zsh.org/cgi-bin/mla/redirect?WORKERNUMBER=4073 (Bug report)
http://www.zsh.org/cgi-bin/mla/redirect?WORKERNUMBER=4099 (Bug report)
http://www.zsh.org/cgi-bin/mla/redirect?WORKERNUMBER=4568

I'm strongly in favor of having zle widget functions return a useful
value (see 4067) and Zefram even agreed with me, but I don't think
anything ever came of it.