If the primary body you are interested supports ephemeris calculation (all the major bodies in Orbiter do) then you can get the barycentre velocity by calling the planets clbkEphemeris function.
EDIT: If the body doesn't support ephemeris calculation you can get an ordered list of bodies (see this thread for ideas on how to do that) then calculate the barycentre velocity from the mass and velocity of the parent object and all its children:
If the primary body you are interested supports ephemeris calculation (all the major bodies in Orbiter do) then you can get the barycentre velocity by calling the planets clbkEphemeris function.
For Earth clbkEphemeris/clbkFastEphemeris return only EPHEM_TRUEPOS|EPHEM_TRUEVEL|EPHEM_POLAR. Not EPHEM_BARYPOS/EPHEM_BARYVEL
EDIT: If the body doesn't support ephemeris calculation you can get an ordered list of bodies (see this thread for ideas on how to do that) then calculate the barycentre velocity from the mass and velocity of the parent object and all its children: