Differential Timing in IBIS

I am wondering how I might "measure" the Output Buffer delay for a
differential output. For a single-ended device, I would use the Rref,Cref,
and Vref load and measure the time
it takes the output to reach Vmeas. However, for a differential output
that swings from -400mV to 400mV, how would I measure the Output Buffer
Delay? A differential output is
defined single-ended in IBIS, so for example an LVDS differential output
might have a Rref of 50 Ohms (100 Ohms differential) and a Vref of 1.25 V.
(The Vos of the device)
Would I use this test load to get the Output Buffer Delay? Can simulators
like XTK and SpecctraQuest "apply" this delay to the differential signal?
In Signal Explorer
(in SpecctraQuest), if I use the single-ended Output Buffer Delay, this is
the same buffer delay that shows up for my differential signal.