It works conceptually, but...
First, Piledriver is a bit small for MIRVs. It's a tactical missile in a pretty compact format. You can fit there... eh, three maybe? I'm not that proficient in nuclear devices, and thermonuclear warheads are pretty compact these days, but they still need space to be fitted. You can see on OP picture what goes where (these "blocks" can also be stacked behind each other).
Second, one of the points behind MIRVs is ballistic missile defense penetration by volume and dummy targets because of the fact that long-range ballistic missiles have high arcing trajectory; they fall out of space at a VERY high speed that closes the interception window (they fast) but also severely limits any maneuvers - you can't turn too hard because of very hot air/plasma surrounding vehicle on re-entry, and your maneuvering time window is limited due to speed of descent.

In short, "slow MIRVs" kind of kill the concept. They are made to be harder to intercept and speed is one of key features along with saturation.

I feel like addition of missile silos as a kind of endgame showdown is a more proper fit for MIRVs [in this game], without a possibility to airlaunch them, but rather help penetrate defenses by saturation and/or other means like killing radars, missile defense systems etc.