I'm pretty sure i know how to do this (just
replace the list-making clause of the "atom:"
production with:

'[' [test [list_iter | ',' testlist]] ']'

then update com_list_constructor and the LSQB
clause of com_atom appropriately), but i still
much prefer a style that uses a separator --
if not between clauses, then at least between
the expression and the conditions.

My favourite: [x*2, for x in spam, if x > 3]

The separator after the expression can be comma,
semicolon, or omitted; the separator between
conditions can be any of these as well as colon.
(Greg's original patch chooses "nothing-nothing";
i choose "comma-comma".)

The results of Greg Wilson's survey may also be
useful.  Once a decision is made, it should be 
easy to adjust the grammar for any of these
twelve possibilities.

So all we need is a Pronouncement. :)