We discussed adding warnings during registrations of custom grid types to make sure users are aware of the portability constraints. Here's a couple of options we could consider:

• Lock down the grid registry at the end of our native type registration. Subsequent attempts to register types would result in an error or warning. An error could be the default with a compile time flag dropping this to a warning to enable customisation of the grid registry from external software.
• Only warn during file serialisation. When serialising custom types, promote a warning that the grid types (and subsequent files) may not be compatible with other installations of OpenVDB.