Hi Matt,
I just checked, and we do not offer an API endpoint for submitting multiple object timeline calculations via our API. The current endpoint ( calculateTimeline) is coded to only accept single IDs, and there was nothing I could find in the backend to get around this limitation. While I wish I had better news, hopefully, it is easily worked around with a for-loop of sorts from your end.
Thank you,
Haven Meyer QA Manager @ Workfront