OK, I think I've sorted this out. Digging in the REST API code I found out this: the querystring should contain "order=-startDate" to obtain a list of the latest 1000 connections sorted by descending start Date (from latest to oldest) The dash character before startDate sets descending order. If removed, the order is oldest to latest, so the default.
So the curl command is like /usr/bin/curl -s -S -H "Guacamole-Token: $AUTHTOKEN" -G -d "order=-startDate" https://$GUACAMOLE_SERVER/guacamole/api/session/data/$DATASOURCE/history/connections to obtain a list of the latest 1000 connections. I hope this is useful for others. Best regards Lorenzo
