Multiple scalar values used as key creates Python hash error

Yes, it's possible, using a custom constructor for yaml mappings.

For example, the following script

import yaml

data = """
? - Detroit Tigers
  - Chicago cubs
:
  - 2001-07-23
"""

def construct_yaml_map(loader, node):
    pairs = [(tuple(key) if isinstance(key, list) else key, value)
             for (key, value) in loader.construct_pairs(node, deep=True)]
    return dict(pairs)

yaml.add_constructor(u'tag:yaml.org,2002:map',
                     construct_yaml_map)

print yaml.load(data)

produces

{('Detroit Tigers', 'Chicago cubs'): [datetime.date(2001, 7, 23)]}