Boxing out floor joists when using 11-7/8 I-Joists?
Since I-Joists are an engineered system, it's vitally important to follow the specific instructions (including the "you can't do that" instructions) of your specific manufacturer for the specific product you have, as they are not all the same. Mine came with at least 4-5 double-sided pages of detailed instructions covering common issues.

I would strongly question the wisdom of following a floor plan layout destined for I-joists which parked a tub drain going through where an I-joist should be. That's poor planning for the construction method. If you are in the "as yet unbuilt" stage (which it sounds like) then redesigning the joist placement or tub placement so that they do not conflict would be the best approach. Generically, a single extra joist and different joist spacing (such as 19.2 inches rather than 24, or 12 rather than 16) in the immediate area should be an adequate solution that does not involve the compromises of mid-span support. Alternatively, a tub with a drain in a different location might solve it, without moving the tub itself.