I believe the mxd field aliases are stored in the mxd document and are not available via arcpy.
To illustrate, I created a simple point feature with a field named "FieldName" and gave it an alias of "FieldAlias". I added it to an mxd file.
In the mxd file, I changed the field's alias to "MxdAlias".
When I examine the feature properties in the data base, the alias still shows as "FieldAlias". However, when I examine the contents of the mxd file, I can see the renamed alias listed along with the original field name.