The current crop of Apple applications are weird to script. I don't use rb-appscript, but here is working code for Applescript that you should be able to alter to taste and port:
property dummyList : {"Tyler Durden", "Marla Singer", "Robert Paulson"}
tell application "Pages"
set theDocument to make new document
tell theDocument
set bulletListStyle to ""
set lastListStyle to (count list styles)
repeat with thisListStyle from 1 to lastListStyle
set theListStyle to item thisListStyle of list styles
if name of theListStyle is "Bullet" then
set bulletListStyle to theListStyle
end if
end repeat
repeat with thisItem from 1 to (count dummyList)
set body text to body text & item thisItem of dummyList & return
end repeat
set paraCount to count paragraphs of theDocument
repeat with thisPara from 1 to paraCount
select paragraph thisPara
set theSelection to selection
set paragraph style of theSelection to "Body Bullet"
end repeat
end tell
end tell
What this does, essentially, is place each list item in its own paragraph (that is what a list item is for all intents and purposes: an indented paragraph with a bullet), select each paragrah in turn, then apply the list paragraph style to the selection. The paragraph
object just returns the text of the given paragraph and does not hold any state in and of itself, for some reason. This isn't the best way to handle this scenario, but at least all the components are there to get you what you need.