Just remove a scavange pipe connection from the sump scavange rail....this will let the sump drain of any oil still in it.
I had to drain my Pace/Brise kidney tank system recently, if the pump is as it should be there won't be much oil drainback to the sump in fact, in which case just undo the lowest pipe connection to the tank and let it drain from there.
My Brise tank has a drain plug but as it's about 3" up the tank it's particularly pointless.
It'd be really useful to have a trolley mounted pump with electric motor.....I was thinking about getting some made, which would allow deep filtration of the oil in a car without pain.....
Just stick the pickup down to the bottom of the sump via the tank cap if drysump, or the dipstick hole if wetsump, switch on, return pipe to either a clean 5 litre container or direct back to the engine, switch on and your oil is deep filtered down to under 1 micron.
It's far to sensible an idea of course, no-one would be interested in using such a thing.