Regular homework remains accessible after the due date -- you have a "check answer" button instead of a "submit answer" button. No recorded grades are updated.
I think that adding an LTI connection doesn't change this behavior. WeBWorK might be updating the grade back to the LMS but since the grade is not changed with additional answer submissions nothing happens.
I don't know how d2l handles homework -- does it deny access to homework after the due date? If so perhaps you can extend the due date for d2l but not
extend the due date for the WeBWorK engine. Then I think you would get the
behavior you are seeking.
WeBWorK also has the "guest" access supplied by the practice1, practice2, ... users. At UoR I make those available so that students can log in to previously
open homework assignments and do a different version than the one they were originally assigned. The results for practice1, etc. a not recorded.
Do you think one of these ideas will work for you?