Apart from keeping an eye on the RP news section, I don’t know.  Mares don’t need to be registered as official broodmares until they are actually covered and covering certificates aren’t usually lodged until later in the year when stallions have completed their book, so there isn’t really a list from which to take this information.  Some very observant person might make it a mission to keep track of any news that has been announced, but I don’t know of a site.
In addition, fillies that have been covered can continue racing until July, and obviously beyond that if the covering has been unsuccessful. ÂÂÂ
If there are individual fillies you are interested in, let me know and I’ll see if I can find out, but I think your best bet would be to check the RP cuttings library.