Dynamic readonly

On my cf7 form I have a email element.
As default it has the user_email
[email* your-email default:user_email]
I’d like to set it to readonly – but only in the case that the user is logged in.

My current approach (workaround) is
to do a hook on wpcf7_form_elements and grep for the email and set the readonly in there.
But I have to be carefully and avoid changing in other forms where this should not happen.

Is there a better solution?
Thx