The plugin converts the url it checks to lowercase, so you should enter the meta value as lowercase.
The URL is exactly as stated. Eg if you have query strings that can variable you would need to treat each one seperately.
There is no reason why you can't have multiple legacy-url meta values associated with a single page. If you have the same meta value associated with multiple pages, the programme will call the first one it finds.
This is a simple solution, and possibly quite inefficient. It uses header redirects so will effectively process one page and then load another.
Because it redirects, search engines will not store the campaign/legacy URL but the page it is redirected to. The more sensible ones (eg Google) will note the redirected page and update their internal databases.
You can change the name of the custom field that the plugin searches by amending the function itself.
Compatible up to: 3.2.1 Last Updated: 2011-11-20 Downloads: 2,671