Those proposals shorten fns by at most three characters. Are multi-arg fns used often enough to warrant this? news.arc contains 23 multi-arg fns in 1769 lines of code; therefore they would save about 1 char every 26 lines.
That would be ok if the proposals were simple and elegant, but personally I find them hackish and inconsistent with the rest of the language. They also don't fully replace fn because they lack an equivalent for (fn args ...).
Here's my idea: just replace 'fn with a special symbol, like \. This seems to work:
--- brackets0.scm 2008-11-11 17:06:01.000000000 -0600
+++ brackets.scm 2008-11-11 17:06:17.000000000 -0600
@@ -18,7 +18,8 @@
; a readtable that is just like the builtin except for s
- (make-readtable #f #\[ 'terminating-macro read-square-brackets))
+ (make-readtable #f #\[ 'terminating-macro read-square-brackets
+ #\\ 'non-terminating-macro (lambda _ 'fn)))
; call this to set the global readtable
personally, I think that (fn (a b) (+ a b)) is more readable than (\(a b) (+ a b)), and readability matters much more than number of characters.
Also, the [:] form could save more characters, if it automatically applied the outer set of parens to the body form.
However, I don't think it's really that much of an improvement; fn works well enough unless you really like extra syntax.
What was it the original poster wanted, anyway? It sounded like something that was more readable than _1 etc. for the var names; thus my dredging of the old thread. If not, then obviously, it wouldn't be a good choice. Maybe the [:] form should be capable of only naming some of the args, and leaving the rest to the other naming convention? Then the  form can name the first n arguments by putting them before a :, and have the args after that referenced by $, $0, $1, $2, etc. or some better character set, if _ looks bad.
It's only a problem if fns of two or more args are common, and they don't seem to be. In news.arc, srv.arc and blog.arc they appear once every 123 lines. In my CL code they appear every 250 lines. Are they more common in your code?