In my experience, no. The terms of the promotion are pretty clear on this. However, I often am surprised when I learn of exceptions made when terms and conditions have not been met for qualifying for a Rewards benefit or status. Our resident rules expert, bejacob might have a better answer to this one.
I'm not 100% sure on this, but it should count.
The terms and conditions state "Members who register by April 15, 2016..." and then go on to say "...for nights stayed between February 1, 2016 and May 15, 2016 at participating Marriott properties."
Now that you've registered, it might be worth keeping an eye on your promotions for the next few days. If the earlier stay doesn't appear, it might we worth calling the customer service line. Chances are it will be there shortly without any intervention on your part.
Keep us posted on what happens. We'll all benefit from knowing.
Of course, bejacob did have a better answer. After a double check of the Terms and Conditions (at least that accompanied my offer), I agree with him that the earlier stay should count. There is no stated condition that prior registration is required. Like him, I had to call to get enrolled in the Mega Bonus, and I might have been in your same situation if not alerted by the MR Insiders of this MegaBonus. Let us know what happens.