From Wee See, Support for importing OpenFlight Surface Material Codes using osg::Object::UserValue.  Notes from Wee See:

See attached my ported code, which fulfills:

- using of setUserValue()/getUserValue()
- user-value will be attached to Geode or Drawable
- actually 3 values will be added: SMC (surface), FID (feature) and IRC (IRcolor). Its UserValue-names are < UA:SMC >, < UA:FID > and < UA:IRC > (without spaces!)
- user-value will be attached to Geode or Drawable
- has an option (named preserveNonOsgAttrsAsUserData) for control whether the UserValues should be attached (Default: no UserValues will be attached)

Reading out this values is very simple. Here is an example (uservalue-name without spaces!):

signed short smc;
if (myGeode.getUserValue("< UA:SMC >", smc))
{
  doSomethingWithTheGeodeAndItsSmc(myGeode, smc);
}
else
{
  // geode has no SMC
}