Hi
Glad to see that it helped somebody.
No I dont think it could negatively affect site indexing. You can perform check on the sitemap.xml directly with Google Webmaster Tools.
I dont know about any other sitemap extension. The only one I use is the Google XML Sitemap, but I guess it could be hacked too.
The purpose of this hack for me was to include attachment page in order to auto cache them with cache plugin like W3 Total Cache or Quickcache. Both of these extension have an option to automatically perform caching of page included in the sitemap. But in the original sitemap.xml, there was no attachment page listed.
Regards
