anybody think they could write a plugin to do this, or possibly point me to one already doing this?

Already did it.
http://www.vendetta-online.com/x/msgboard/3/22561#280200

ignoreturret.zip
bind ignoreturret instead of normal target in front command
ie.
/bind b ignoreturret

there's also /toggleignoreturret

I suppose it'd also be nice to do the same for closest target or whatever that command it?
Or for all of them?
Say if you want me to add/change that or anything else.

thanks