Here are my four ways to gain negative camber on Sprinters and any other Toyota using a similar "Macpherson Strut" front:
1) Just as you suggest by lengthening the lower control arm. There is a "thread" about control arms modified to use "rose joints" which allows a wide range of camber adjustment and better than a "cut and shut" extension. I know of no offset bushes available.
2) Redrill the mouting hole location on the crossmember and perhaps weld steel plates in, to allow the hole to be moved even further out.
3) Purchase a suitable Adjustable top for the strut such as the ones from K-Mac.
4) Bend the strut at the lowest point.
A good set-up I use for my race car (TA22) is 2 degrees negative and as much positive castor that can be gained from the radius rod.
Option one and two would be the best for drifting. The increased track will be benificial, but may push the wheels out past the guards.
So option three is the next best.
Option four will result in decreased clearance between the tyre/rim and the strut and will be worse if you use spring platforms.
Or combine the first three to give the greatest flexability.
I use option two in my "avatar" TA-22 and still needed to bend one strut to match and get the 1.5 negative on both sides.
I used all four in the race car and will again in the new shell.