Disregard, I think.  I'm not sure why, but my current app seems to be doing the converting.

>>> import plistlib
>>> pl = {'My key': 'False'}
>>> plist = plistlib.writePlistToString(pl)
>>> plist
'<?xml version="1.0" encoding="UTF-8"?>\n<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">\n<plist version="1.0">\n<dict>\n\t<key>My key</key>\n\t<string>False</string>\n</dict>\n</plist>\n'
>>> plistlib.readPlistFromString(plist)
{'My key': 'False'}
>>> pl = {'My key': False}
>>> plist = plistlib.writePlistToString(pl)
>>> plist
'<?xml version="1.0" encoding="UTF-8"?>\n<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">\n<plist version="1.0">\n<dict>\n\t<key>My key</key>\n\t<false/>\n</dict>\n</plist>\n'
>>> plistlib.readPlistFromString(plist)
{'My key': False}