You can drain the oil tank and then remove the oil filter and you will not loose any more oil. The reason for this is the oil pump design. It will not allow any significant oil volume other than a little oozing to come through the pump. If oil could flow through the pump on its own then there would be no reason to turn the prop to turn the oil pump during an oil purge. You can test this by draining the tank and removing the filter after it is drained. Then try it your way. The small oil loss at the filter will not change. Then when you put the oil and filter back in play and "Vent" the system by rotating the prop it will fill the oil filter and help purge the small amount of air in the system. The air introduction at this point is small. I typically have my new oil filter ready to screw on when I remove the old one to just help keep the oil drool mess to a minimum.
Here's another thing I learned on the Rotax forum. When you want to pull and clean the tank you can disconnect the fittings on top of the oil tank and just put a rubber stopper in the so they don't lose any oil if they hang down. The system will not lose any more oil than a regular oil change because the oil drains from the fittings anyway. Then you can replace the tank, connect the oil fittings and fill the tank with oil and do a vent like you do for a regular oil change.
No reason for an oil purge. Purging and venting are two different things as spelled out in Rotax SB's.