For anyone else who may be looking for a solution to this I made a function that allows an input box with an up/down control to contain a value greater than 32767.
Func _GUICtrlUpdownSetRange32($h_updown=0, $i_Low=-999999, $i_High=999999)
; Windows message ID for the UDM_SETRANGE32 message
; UDM_SETRANGE32 - Sets the 32-bit range of an up-down control.
; http://msdn.microsoft.com/en-us/library/bb759968%28VS.85%29.aspx
Const $UDM_SETRANGE32 = $WM_USER+111
GUICtrlSendMsg($h_updown, $UDM_SETRANGE32, $i_Low, $i_High)
EndFunc
This sets the range to a default of -999999/999999 but I believe the Low and High values can be any value less than (2^32)-1.