It could very well be the waterpump seal. There should be a weep hole on the case right below the waterpump(on the housing-rather tiny hole). If the seal has failed you should be losing coolant there also(that is why tthe weep hole is there, to be an indicator of the seal's integrity). I am not sure on that model, but usually there are two seals behind the waterpump impeller, one holds coolant on its' proper side and trans oil on its' own side. The weep hole will be between the seals. It is a fairly easy job to change the seals, but I wouldn't try to do it without removing the right side case cover(the inboard(oil) seal is usually installed from the inside).
Another possibility is that the waterpump housing has eroded and is now porous. This will necessitate replacing the housing/clutch cover.
Your overnight 'test' of the trans oil is good thinking, but unless you let the old oil drain for that length of time as well(at least), it isn't very conclusive. The clutch pack alone holds back a signifigant amount of oil.