Giving info to CONSTANTP

I have a function that produces constant results when given constant arguments, and I would like to make CONSTANTP aware of that, so that:
(constantp '(my-function 5 8)) -> T

The CLHS seems to say the treatment of that kind of expression is implementation-dependant. I'm using CMUCL; I looked into the docs and found only:
(declaim (ext:constant-function my-function))
which looks like it could have helped, but CONSTANTP doesn't seem to care.

  (constantp '(+ 2 3))
gives NIL on CMUCL, so I'm not expecting much...

Any ideas?

PS: I'll also take ideas about achieving this:
  (constantp '(+ 2 3)) -> T
.