multiline.py

import yaml
from collections import OrderedDict

class quoted(str):
    pass

def quoted_presenter(dumper, data):
    return dumper.represent_scalar('tag:yaml.org,2002:str', data, style='"')
yaml.add_representer(quoted, quoted_presenter)

class literal(str):
    pass

def literal_presenter(dumper, data):
    return dumper.represent_scalar('tag:yaml.org,2002:str', data, style='|')
yaml.add_representer(literal, literal_presenter)

def ordered_dict_presenter(dumper, data):
    return dumper.represent_dict(data.items())
yaml.add_representer(OrderedDict, ordered_dict_presenter)

d = OrderedDict(short=quoted("Hello"), long=literal('<style \n      xmlns="http://purl.org/net/xbiblio/csl"\n      class="note"\n      version="1.0">\n  <info>\n    <title>Test fixture</title>\n    <id>http://citationstyles.org/tests/fixture</id>\n    <link href="http://citationstyles.org/tests/fixture" rel="self"/>\n    <link href="http://citationstyles.org/documentation/text" rel="documentation"/>\n    <category citation-format="author-date"/>\n    <updated>2014-04-30T13:19:38+00:00</updated>\n    <rights license="http://creativecommons.org/licenses/by-sa/3.0/">This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 License</rights>\n  </info>\n  <macro name="author">\n    <names variable="author">\n      <name form="short" />\n    </names>\n  </macro>\n  <citation>\n    <sort>\n      <key macro="author" />\n    </sort>\n    <layout delimiter="; ">\n      <text macro="author" />\n    </layout>\n  </citation>\n</style>'))

print(yaml.dump(d))

I know this is very dead but I don't have enough rep on stackoverflow to answer your comment.
The reason this doesn't get quoted as a block literal is that PyYaml ignores the style="|" argument if your string contains adjacent newlines or whitespace before or after a newline. PyYaml dumps it as a double-quoted scalar instead.
yaml/pyyaml#121

Thanks @johnhaire89 ! This would have taken a long time to figure out!