Hi,
Yes, Adobe will support at.js via DTM, but I don't have specifics around the timeline. In the meantime, I believe you can deploy via custom code section in the Target tool. You would need to download the at.js code from Target, open your Target tool in DTM, past the at.js code in the custom section, save the changes, and test via your staging environment. You might also confirm with the Target team by emailing them directly via ttclientcare@adobe.com.
Thanks,
Jantzen