So today it took me a few hours to test out a proof of concept: to have foamToVTK in OpenFOAM-dev to work properly with Int64 and Double precision datasets.

The resulting code is in this commit: https://github.com/wyldckat/OpenFOAM-dev/commit/0404fe18bad538a9d26ea00855dedcfbacd744fd

Long story short:

1. double precision is indeed supported, so this is a viable data format that can be used in the legacy VTK formats. This is detailed here: http://www.vtk.org/wp-content/uploads/2015/04/file-formats.pdf
2. vtktypeint64 was introduced back in 2007: https://github.com/Kitware/VTK/commit/d6ef8ceac286e1f39ff6df96132ba746d2e5598e
   • Fun fact: the 64-bit integer datasets were probably available as of ParaView 3.4.0, given the VTK timeline vs ParaView timeline: http://www.paraview.org/Wiki/ParaView_Release_Notes#ParaView_3.4.0_.28October_2008.29
   • But there is big a problem with it: it's only for datasets, it is not meant to be used for the unstructured mesh itself. The CELLS entry is hardcoded in VTK to handle only 32-bit integer binary data.

The only real solution is to start using the XML VTK format, which requires creating a new data exporter, or to rely on binding to VTK for direct data loading and processing.

Problem is that at the time of this writing (2016-02-20) it's still not exactly viable to go forth with such a forward thinking implementation, unless someone is truly interested in funding the implementation.