1) struct _CONTEXT has unused 1a0 bytes, it is enough to store high part of ymm0-ymm15 (low part of ymm* is xmm* and stored by kernel itself)
struct _M128A VectorRegister[26];// Offset=0x300 Size=0x1a0
2) struct _KERNEL_STACK_CONTROL has embeded _XMM_SAVE_AREA32 union, no enough space to store high part of ymm0-ymm15 regs
I dont sure what struct used on x64 to store FPU context, if _XMM_SAVE_AREA32 - need to expand struct (200h->300h) and change all related size/offsets in kernel, finding these places may be very hard, see KxContextToKframes(), KiInitializeKernel(), KiInitializeContextThread(), PspGetSetContextInternal(), RtlInitializeContext(), KeSaveLegacyFloatingPointState(), ...