As a follow up, I tried this and the TRY/CATCH causes the ALTER TABLE to fail in the same way that having an ON ERROR routine enabled. I'm stumped.

        lcWasOnErr = ON("ERROR")
        TRY 
          ON ERROR
          ALTER TABLE myCursor ADD COLUMN NewField C(10) 
          ON ERROR &lcWasOnErr.
          =MessageBox(lcAlias + ": Added field '" + lcField + "' .")
        CATCH
          ON ERROR &lcWasOnErr.
          =MessageBox("*ERR* Failed to add field")
        ENDTRY