Is it possible to modify a pulse schedule of a circuit compiled using QASM? I know this is possible using OpenPulse, but it seems none of the IBM backends support OpenPulse. So is it possible to do this through QASM? This paper gives me the impression that I would only be able to modify the QASM command order, but not the actual pulse schedule. Is this correct?
If you mean at the circuit schedule level, then yes the transpiler supports scheduling a circuit and resolving all timing by inserting appropriate delays on all qubits according to the selected scheduling policy. See the transpiler option scheduling_method here. As KAJ226 notes, for more explicit control you may use the pulse level Schedule representation which may be obtained as shown in this tutorial. However, only ibmq_armonk currently supports public pulse access.