I, too, caution against the use of limestone as a compost component. From what I've been told
by those who have seen many species of Nepenthes in situ, the mere fact that a plant is growing
on a limestone outcrop does not mean that it is growing
in limestone. Actually, plants
are purportedly growing in layers of humus which collect on such formations.
As an aside, some time ago, it was speculated that limestone might be a useful addition to the
compost of N. northiana. However, I know of no one who was successful growing this species
in compost with this component. In fact, I've grown this species to maturity (with leaves
well over 2 feet in length) in a compost utterly devoid of any limestone whatsoever.
You can see a somewhat dated photo of a pitcher at
Plants with Attitude - northiana. Just my two cents worth....