> fn bt {
> 	eval '{'for {tail -f $i &} '}'
> }
> 
> mimics how I would write it by hand.

if that's all you need, then

fn bt {
	for(i){tail -f $i&}
}

should be just fine.
then

	bt > file

might work (if file is append-only, otherwise you'd need a process
(e.g.  cat) in the middle to avoid overlapping writes)

using eval is disastrous...  consider what happens if you've got a
filename with a shell metacharacter in it.