Hi!
I have been working in E-Commerce since just above 3 years now, and have been solving many such issues related to xml creation, sitemaps, and xml feeds. I distinctly remember solving an issue my website faced sometime back which is stark similar to the one you have described.
I am very well versed in Open Source E-Commerce Platforms, PHP, SEO Tools, Google Webmaster Tools, Google Analytics Tools (including creating monitoring reports, scheduled reports and more), Universal Analytics and more.
Have worked on all these in the recent past for the E-Commerce Firm that I am handling as of now.
I would love to discuss with you further on the same, and if it the same issue that I recently solved then I could get this done really fast.