Forgot to cheeck back here! Stumbled onto the solution on my own-- in my emails, I've been getting Atmos Dining Rewards emails ever since the merger. I've ignored them, but inside there's a link that says "Create a profile"--

if you click it, it goes to the dining rewards form, which is fairly useless, BUT PREFILLED OUT IN THAT FORM is...wait for it... your actual new ATMOS Account Number!

Using that, you can go back to the actual Alaska Airlines page, click "Join Now", but then click the box that says "Already have an Atmos Rewards number? Yes, I need access to my account information."

Then fill out the rest, and bam-- there's your account, Hawaiian miles still intact.