This is Bhushan from PTC Technical Support.
On latest Windchill REST Services, we have a provision to run Info*Engine Tasks using REST Service: /Windchill/servlet/odata/v1/IE/InvokeIETask
This can be leveraged to generate next sequence number, however currently Windchill REST Services do not have provision to create objects with user input numbers where Auto Numbering is enabled.
If you need this next sequence number to be reserved for any other custom operation, you can develop an Info*Engine Task and run it using Windchill REST Services.
Please let me know if this helps.