You might be able to emulate it somewhat with Automatic Groups.
You could create an Automatic Group with no members. Take an Action targetting the Automatic Group with “Action Start Time” set in the future, and “Begin downloads before constraints are satisfied”.
Modify the Group’s relevance to add clients to the group when you want the downloads to begin. Once the client is added to the group, they’ll become Relevant for the action and begin downloads. When the Action Start Time arrives the clients will begin executing.
If you modify the Group’s Relevance to kick computers out of the group before the action start time arrives, they’ll stop being relevant for the action - but if their client cache folders are large enough, I think the downloads should remain cached. So you could add them back into the group later (the next night) to let them resume downloading.
I haven’t tried it myself, so take that with a lot of salt, but it might approach what you’re trying to do.