It is known that the Sefer Halakhoth Gedholoth codifies that since only the descendants of Aharon HaKohen were strictly commanded regarding netilath yadhayim with a keli, it is therefore enough for a yisrael to simply immerse their hands/fingers in a vessel containing water without the need to pour it over the hands.
I have also read that in a sha^ath ha-dohaq one may use a faucet to wash before prayer.
Are there any other sources from Geonim or Rishonim that permit washing for prayer without the use of a keli? Are there sources that discuss the difference in obligations for washing as they apply to a kohen or a yisrael?