Thanks, everybody! Lots of useful advice there.
Based on one of Murray's suggestions I'm doing this:
# app/views/articles/show.html.haml
:markdown
#{assetify @article.content}
# articles helper
def assetify(text)
text.gsub(%r{(/images/news/([-a-zA-Z.]*))}) { |name| asset_path("news/#{$2}") }
end
It does the job.
Cheers,
Andy