You can use SEO4Ajax [1] to crawl your SPA and generate an up to date sitemap dynamically.

[1] http://www.seo4ajax.com/

We're using Brombone. You give them your sitemap, they do the prerendering and save it all. Then you proxy to them for Googlebot and others.

Dynamic constructions of sitemaps is surprisingly difficult. We would need to poll every single page you have, just to check if you potentially have a new link to a new page in your site. And everytime you add a new page, that's a whole another page to scrape and analyse.