I spoke to Solarwinds support who advised i make a request here.
There exists average-time-to-assignment, and many other average metrics that have business hours options such as "average time to first response (elapsed time)" and "average time to first response (business-hours)" but not for time to assignment.
This would be great for time elapsed measurements that are only in the business.
Our business aims to assign and review tickets within an hour of them getting raised, but a ticket raised on Friday at 6pm won't be reviewed until Monday at 8am, thus tanking the metric.