So having a quick look, I don't believe the XML has changed, however you now need to register to get an OAuth consumer key to be able to access the API and the number of requests may be limited from the information I was able to gather. The actual code for gathering the weather resides all in the core WMC program I just call functions to display it in my themes. So I believe to get this functional again the admins would have to register an app with yahoo and then they wil send an email asking for access to yahoo-weather-ydn-api@Oath.com. After which it would just be a matter of redirecting the call within WMC core to the new RSS feed as it still seems to use the WOEID to identify the location.
Thanks also from me for looking into this.
Only a couple of days ago the site where I previously found my local Yahoo WOEID reported that it had closed. That site is at http://woeid.rosselliot.co.nz/. Today, when I opened the site to check on its status so I could report it here now states "WOEID Lookup Service is RESTORED."
So it, at least, is working again.
It would be great to see the weather panel working again in Emby for WMC.