You can seek to propose an initiative and file it with the Attorney General. If you obtain enough signatures, it can be placed on the ballot.
You can also speak to your legislator and ask him or her to craft such legislation.
However, a notarized statement does not show medical need, only that the notary saw the ID of the person that signed the document.
Also, CVC 26708(b)(10) already provides for a medical exemption:
(10) Sun screening devices meeting the requirements of Section
26708.2 installed on the side windows on either side of the vehicle's
front seat, if the driver or a passenger in the front seat has in
his or her possession a letter or other document signed by a licensed
physician and surgeon certifying that the person must be shaded from
the sun due to a medical condition, or has in his or her possession
a letter or other document signed by a licensed optometrist
certifying that the person must be shaded from the sun due to a
visual condition. The devices authorized by this paragraph shall not
be used during darkness.
Though the device must be removed during darkness.