Does there exist a service that will allow me to create a twitter search (i.e. "find all tweets with the tag '#aardvark' that are emitted by someone whose name contains the string 'pete'") and then "subscribe" to this query (for instance through RSS)?
Twitter Advanced Search has some of what you're looking for -- f'r instance, here's the feed for tweets with the tag '#aardvark' that are emitted by someone named 'pete' (which has no results, sadly)
Unfortunately, there are no wildcards that I'm aware of, so it's an exact-match on the tag and name.
There are a lot of other options available, though.
Perhaps it might be possible to magic-up some google-fu to do this -- and then set up a google-alert for the RSS feed.
Again, I could not find a way to limit it to a specific sender, nor to hashtags (since the "#" is ignored)