Preliminary research! Do not pay much attention to this.
0x9A - Matrix letter input dialog
0x197 - makes emulator screen become quite dark. called on PowerOff and Restart.
0x8E8 - InputDateDialog
0x8E9 - InputMonthDialog
0x8EA - InputDayDialog
0x8EB - InputYearDialog
0x183C - makes emulator screen go completely black. OS keeps running fine behind it. Emulator reboots will not bring the screen back on. Called on PowerOff.
Not completely syscall-related, but anyway: On emulator with OS 01.02 and probably on every calculator (since this knowledge was acquired from REing OS 02.00 dump):
0xFD8014AC is a pointer to the address where to store the backup of the first message box pushed (confirmed) 0xFD8014B0 is the same thing, for the second message box pushed (i.e., push one msg box then push another) (confirmed) 0xFD8014B4 might be the same thing, for the third message box pushed. not confirmed