Jon - thanks, you set me on the right track -- the app I want to paste into (MarginNote) accepts rich text only if it's formatted as an Apple Web Archive type. Here's how I ultimately got it to work for Python 2.7, thanks to this guide on coercing HTML into an archive type:
from objc_util import *
import os
import base64
theDefinition = '<html><head></head><body>''<font size="5">' + '<b>Hello</b> <i>world!2</i>' + '</font></body></html>'
theData = base64.encodestring(theDefinition)
theData = str(theData)
theArchive = """<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>WebMainResource</key>
<dict>
<key>WebResourceData</key>
<data>
""" + theData + """ </data>
<key>WebResourceFrameName</key>
<string></string>
<key>WebResourceMIMEType</key>
<string>text/html</string>
<key>WebResourceTextEncodingName</key>
<string>UTF-8</string>
<key>WebResourceURL</key>
<string>about:blank</string>
</dict>
</dict>
</plist>"""
thePasteBoard = ObjCClass('UIPasteboard')
thePasteBoard = thePasteBoard.generalPasteboard()
theType = "Apple Web Archive pasteboard type"
thePasteBoard.setValue_forPasteboardType_(theArchive,theType)
