You can have a look at a similar question that has been answered by MathWorks Support Team here: Link
For your specific query, follow these steps:
blk_lines = get_param('model/subsystem','LineHandles');
Then use the 'DstBlockHandle' property of the outport lines to get the destination block handles. For the full path of these blocks, use 'getfullname' on the handles. (refer getfullname)
plant_handle = getfullname(get_param(blk_lines.Outport,'DstBlockHandle'));
Here, if your chosen subsystem has multiple outports connecting to multiple blocks, 'plant_handle' is a cell array with the elements being character vectors containing the paths of all the destination blocks. If there is only one outport connecting to a single block, 'plant_handle' is a character vector containing the path of that destination block.
Now that you have the path(s) of the destination block(s), you can get any object parameter of any of the destination blocks using the required property in the 'get_param' function.
For example, for obtaining all 'ObjectParameters' of the first destination block path in the plant_handle cell array: