The 1.5ms is only for periodic frames. I think a trigger channel would resolve this, as this should be implemented as an "event" frame. You would have to trigger it yourself at the period you want.
The NI-XNET hw is not a direct replacement for NI-CAN hardware. It is a new platform with a new API. However, we have provided a compatibility layer that will make most of NI-CAN code work with on NI-XNET hardware. We did also ensure that the NI-VeriStand CAN implementation would work with NI-XNET hardware.