Since you're looking for users and not necessarily tweets, you can scrape the company names you're interested in via a spreadsheet, and then append this URL to each unique name: http://twitter.com/#!/search/users/

Just getting rid of duplicate company names should cut down the list some, but it's still going to be a tedious process.  At least with the spreadsheet you can add further columns to prioritize your work and go after the companies that are most applicable.

To cut the tedium, Mechanical Turk could then process the results for you fairly quickly.

If you're planning on doing this with a non-disposable, branded website it's a bad idea as there are can be lots of negative effects.

If you're planning on being black hat, having a network of disposable sites, and are going to be masking (or trying to mask) everything you do, it's just another tool in your toolbox.

Like people already mentioned here, it's better to make worthwhile comments that can be traced back to a reputable looking source as those could even have the added benefit of bringing you additional business, not just a boost in the rankings.

If I was only giving 8 hours I'd spend it all on project mapping.  The work a site requires to be effective is way more than 8 hours but you could construct a broad project road map that would lead you consistently in the right direction for a year or more.

With the above context I'd be working on increasing domain authority and auditing site architecture to get the most bang for the buck.  Steve's suggestions are spot on.