You can write your own MATLAB script to trace an output signal to its input using get_param function and the handle of the signal. You can do something like:
% example block name
block = 'model/subsystem';
% get block handle by name
blockHandle = get_param(block,'Handle');
% get all port handles of the block
portHandles = get_param(block,'PortHandles');
% get block inport. May need to index if there are multiple inports
inportHandle = portHandles.Inport;
% get input line handle that goes into the port
lineHandle = get_param(inportHandle,'Line');
This can be done recursively until hitting a termination point that can be specified.
