Can I use a dictionary variable to supply all task parameters?

With Ansible 2.2 you can still use args parameter to achieve that.
But it is deprecated for a while already and will display a warning for you.
Some details about deprecation.

Example:

- hosts: localhost
  gather_facts: no
  vars:
    args_list:
      - content: hello world
        dest: /tmp/test1.txt
        mode: 0666
      - content: test test test
        dest: /tmp/test2.txt
        mode: 0444
  tasks:
    - copy:
      args: "{{ item }}"
      with_items: "{{ args_list }}"