Is there a plugin that will show the results of a Twitter hashtag search either as a a widget or in a post/page via a shortcode? The shortcode is preferred.
There's Tweet Blender, seems pretty flexible and full of options:
Provides several Twitter widgets: show your own tweets, show tweets relevant to post's tags, show tweets for Twitter lists, show tweets for hashtags
A widget for displaying the most recent twitter status updates for a particular hash tag.
A widget which allows you to specify and display a Twitter search (or a Twitter hashtag) in your sidebar. Twitter searches are very flexible, and you can display anything from Hashtags to individual, or aggregated Twitter streams.
Hope these work. To add shortcode support would be pretty easy for any of the developers of these plugins, so it would just be a matter of asking them to do so.
You could use just the RSS widget and the hashtag feed. For example: