Solution:
This WP plugin Redirection – Manage 301 redirections without modifying Apache | Urban Giraffe has a CSV import function, so you can import a list of your URL changes and stay out of a long .htaccess file. It logs redirects, 404’s, etc.
You will take some sort of SEO hit, but Google will reindex quickly if you verify your site with WTools and then turn up the crawl rate.
I’d take the plunge and run WP under php rather than a side-by-side install. You might even try a WP demo install with that plugin and throw some test .asp URL’s at it.