A product page template is simply created by making a Product.aspx page and adding it into its own directory under the bvmodules/producttemplates directory.

How to set it the new template as the default is something that I am also looking to do that I have yet to find an answer for. Each product has a product property that lets you choose what template you would like to use, so you can "conditionally" choose between templates on a product by product basis.

If you are talking about making a certain template display on a certain set of conditions. I'm not sure how this is any easier than just setting it up in the product admin, but the great part of bvc5 is you can basically do anything if you have a developer license. Just change the logic in the BuildPhysicalUrlForProduct function in the Core and you rewrite your URLs out to any template you want on any condition/methodology you want.

Haven't really looked into doing it, so I'm not 100% sure its possible, but I guess you could technically create a Product Template that handled the redirect rather than doing it in the core. Then you'd just set all your products to use that one template and that template would decide where to go.

A better way to handle the situation, if you don't want to modify the core, which if you own a developer's copy wouldn't be very hard to apply your code update to each successive sp release would be to write your own http handler to handle the URL rewriting and modifying the web.config to use your handler instead of the one in the bvc core.