I'm trying to transfer an existing .NET site into Umbraco and trying to use the umbracoUrlName to set the URLs for pages to map to their existing URLs so that inbound links still work and it doesn't affect SEO. I'd really rather avoid forwarding, but the problem is that some of these pages are in different folders and umbracoUrlName seems to ignore slashes.
You are right, umbracoUrlName ignores slashes. You will either have to put redirects in place using something like the 301 Url Tracker or place the nodes inside other nodes to simulate the folder structure.
Alternatively, you could use umbracoUrlAlias which does accept forward slashes. This doesn't change the original url, but it does give the page an alias that can also be used to access the node.