if the water is too low in Greenwood creek, the park may allow fly-in access to Mack Lake - they've done that a time or two.  (note that there will be a new supervisor this year, so he may have different views on that than previous supervisor had)

With such a dry winter and maybe continuinig with a dry spring, even in May there is potential for Greenwood Cr to be too low (going to depend on spring rains) - Four ? years ago or so, all water levels were very low in Quetico after a similar but wetter winter - big lakes were down about 3 feet when I went at end of May begin of June.   But as mentioned above, the Wawiag should have plenty of water - I do remember a sorta campsite on the river - there was a fire ring and enough usable space at the downstream end of one of the portages for a "make do" camp if needed - it would be one of the marked portages around a rapid.

If you have an RABC, you can access Wawiag vis Saganagons to Mack Lake if you are looking for other options.