It is possible to use RORB for extreme floods up to the AEP of the PMP. Currently you can alter the IFD data (through the “View/edit IFD data” button) such that you can input any AEP, the only limit is the total number of AEPs specified. The rainfall depths rarer than the 1 in 2,000 though you would need to calculate yourself as the BoM only supply up to the 1 in 2,000.
One thing to be careful when doing extreme floods through RORB is the use of ARFs. RORB does a simplified method of applying ARFs for events rarer than the 1 in 2,000, there is more detail on this in the release notes.
As for Monte Carlo analysis, it is possible to do Monte Carlo Analysis up to and including the AEP of the PMP but you would need to input rainfall depths for an AEP that is rarer than the AEP of the PMP so that RORB can sample either side of the AEP of the PMP.