You can use the method you mentioned and add item 6 to the Brands parent list. Then you would have to create a constructed list at Q1 and not display item 6. Item 6 would always be blank at Q1 as you indicated.
Another approach is to create a new parent list at Q2 and make it the same parent list as Brands but add Item 6 to it.
We then create a constructed list using some Perl script as follows ...
Begin Unverified Perl
for($i=1; $i<=5; $i++)
This script assumes Q1 is a row direction grid question. If Q1 is a select question, simply change the following ...
I have also assumed your parent list at Q2 is named "Q2List". Change this to your list name.
And if you want to randomise the Q1 items at Q2, add in this command above the last ADD command ...