Just donating BE isn't profitable for Riot, and would be easily exploitable by 3rd parties. Being able to give gifts with BE however would be a nice thing to see, with a mark up most likely. The only downside I see in this, is that I would be very easy for long time players to make smurfs with generous sized champion pools. Though I don't play in low ranks I could see how it could effect lots of people in bronze/silver/gold if the accessibility to smurfing is easier.
The other option would be to allow players to buy RP with BE, much like how WoW allows players to buy subscriptions with enough in game gold.