doable but difficult
The Sand Brook route crosses private land and for that reason alone should be avoided. However, you can make a legal approach if you continue up the Marcy trail to state land then roughly contour across (one deep gully at Marcy Brook) then up to the Allen/McDonnell col. Mark L and I did this route in September of '00 and found it to be exhilerating (i.e., extreme). It took us 5 hours to make the 2 miles to the summit, then after a long summit break we made the return trip in the same time of 5 hours. I'll preface by saying we weren't trying to set any speed records that day, as this was my first serious hike after a knee injury earlier in the summer, but still our time was slow - about 18 hours round trip (5:30am-11:30pm as I recall). The good news is we met two great new hiking buddies on the summit of Allen and my knees didn't act up.