I see a couple potential issues. (Better terms for whether the reader macro evaluates subforms might be "evaluating" and "quoting".)
If the elements inside the syntax are permitted to be non-self-evaluating objects such as symbols or conses, then under the evaluating option one would need to write [1 'x (+ 40 2)].
Second, as I'm sure you realize, the evaluation model at read time is very different from regular evaluation in that it always happens in the read-time dynamic environment and the null lexical environment. This doesn't matter if the computations are just simple arithmetic, but it makes read-time evaluation feel messy to me, especially if someone might later use it with more-complex forms.
You might also consider inside your reader macro borrowing the syntax and meaning of the backquote comma:
[ 1 x ,(+ 40 2) ]. This might make the meaning and environment more evident to experienced CL programmers (although comma forms evaluate at run time in the current lexical environment).