When do about-to-be-released games become available for creating giveaways?

You can't buy it. Which is probably the reason.

Games only appear in the giveaway list after their release (normally within 24 hours), to create a giveaway before that you have to submit a ticket in the "Add Game to List" category, and as long as a Steam store page exists, moderators should be able to manually enable it.

To clarify, generally within 24 hours of being made available for purchase, not necessarily the release day.

We may be able to add games before it's available for purchase, but the price will be 0p.

where did you buy the preorder copy?
i don't have an option on steam :O

edit: ok, read the steam discussion forum -.-