For example, where the top level is a net flow of the next two levels? Say net flows into a region - arrivals less departures?
Experiments on a simple 2 level, 3 series, hierarchy suggest no.
All series forecast with positive values.
Setting one base level series (departures) to be negative, but applying a positive summation matrix/nodes, results in very large deviations from the underlying series in the reconciliation process.