Q: BARON reports the best feasible point found (UBD in a minimization), as well as the “best possible” value of the objective (LBD in a minimization) in the output. For my application the latter (“best possible”) is needed. However, I did not find a way of accessing this from within a GAMS program.

You can use the model attribute model.objest. Here is a working example:

option nlp=baron;
$call 'gamslib chem'
$include chem.gms
parameter bestpos; bestpos=mixer.objest;
display bestpos;