The G38 hub is already quite long...If 1/2" longer will work, go for it...Just be sure you perfectly balance everything...
Don't go much longer than that...
Any out of balance condition causes the end of the hub to try to rotate like a bicycle crank..You can sometimes see it when running at WOT...If there are two bearings on the front of the crank the stress is transferred to the bearings...With only one bearing the crank tries to flex in the middle at the joint...The distance from the front bearing to the end of the hub kinda determines how long the hub can be...