Yes, this is a gap. There is no ONTAPI to perform snap reserve, snap schedule, etc. at the aggregate level. The only snapshot related API which you can use at the aggregate level is: "snapshot-list-info" (by specifying "target-type" as "aggregate" and providing the intended aggregate name with "target-name" tag).
[Note: These options (target-name, target-type) are available ONTAPI 1.5 onwards]