You could mount the cooler with the fill/return lines down which would allow it to drain up to the point where you reach level with the engine oil return.
The option of purging the lines is not out of the question. There are a few options out there for adding a fitting to purge the lines.
Most cars I have seen with oil coolers typically have the fittings up and there appears to be no concern about the quart or so that remains.
If you knew that you had done a good job of cooling the oil it wouldn't be too difficult to disconnect the lines at the sandwich adapter and purge the cooler.
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16 Cayman GT4, Carmine Red, Buckets, PCCB etc.
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