I don't see anything special about Lentils as a yeast food, but have cut down on the amount used. Only 100g /25 litre wash these days, + 50g of Sultanas or raisins, it works for me.
I don't see a problem with using your bean mix.
I did sort through the cupboard looking to try something else, but sadly used the Chick peas, which were next up, as baking beads last week, so am unsure if they will still work as yeast food.
They make great baking beads when baking a blind pastry base.
I may try using Couscous as yeast food, I have a box of that, that I will never finish.
Life is a Bagatelle.
Robert.