Yeah. It's a trade-off in amount of code I have to change. From what I've read (and please correct me), everyone accelerates the base instead of applying a gravity force at each node because it has a lower computational cost. That trick doesn't work in my case with the added-mass. 

I have three options then from what I can tell: 
1) Modify the loop to apply gravity at each node (and slow down everyone else's code, since I doubt many people will be using added-mass). Then I also have to create a way to make gravity forces in the RigidBody class, or a new force element.
2) Add a bunch of conditionals in the featherstone loop, so that you can choose between using a gravity force or using an accelerating base. 
3) Keep as is, and use the anti-gravity correction. I think this has the same computational cost as 1&2, since I'm added a correcting force at every node. Seems best to me, as now I'm not mucking with as much of other people's code.
Thoughts?