Hello Chris,
It is not possible to make this function filter by any Timesheet fields.
As I workaround, may I suggest that you create two Custom Fields on User entity to store these sums - one for overtime hours and one for non-overtime hours.
The fields would have to be of Duration type >> Field value is based on a formula of: Relations Summary >> Summary Type: Sum >> Relation: $ReportedTimesheets.Duration >> Filter Related Items:
Use this one for non-overtime hours: TargetObject.Overtime = False && TargetObject.ReportedDate > Date(2017, 4, 1) && TargetObject.ReportedDate < Date(2017, 5, 1)
Use this one for overtime hours: TargetObject.Overtime = True && TargetObject.ReportedDate > Date(2017, 4, 1) && TargetObject.ReportedDate < Date(2017, 5, 1)
Dates can be changed into something else.
Please let me know if this answers your question.