Thanks Seanas - it didn't work but it may have been part of the problem. I eventually ran it down to part of the the AR1100.bcs script never running. Here's the relevant code first:
IF
	PartyHasItem("CBLTCNT1") // Letter of Introduction
	PartyHasItem("CBLTCNT2") // Broken-Sealed Letter
//	!PartyHasItemIdentified("CBLTCNT2") // Broken-Sealed Letter
	Global("CbPlayerCanGoToSoubar","GLOBAL",0)
	Global("CbMayorAndRolfCutscene","AR1100",0)
THEN
	RESPONSE #100
  SetGlobal("CbMayorAndRolfCutscene","AR1100",1)
  CreateCreatureOffScreen("CBUHCUT1",0) // No such index
ENDNote the line I've commented out. The problem seems to be that the moment you pick up the second letter, it becomes identified even if you don't open it. Move your cursor over the top of the unopened letter and wait for the tooltip; it says "Broken-sealed letter" but investigating the cbltcnt2.itm file shows that the general (unidentified) text says "Sealed letter". Commenting out the identification line allows the cutscenes to start running.
I even tried picking up both letters and heading straight for Umar without looking at or reading either one; the script still failed to run. Yes, I do have the soubar_101 patch installed! - so I have no idea why this should  be happening.
-Y-