Seeding new filter media . . .

[hwscot]

So instead of maturing the new sponge in the old filter for two weeks, I considered simply washing the existing old sponge out in a jug of tank water, and then soaking the new sponge in the brown mulm water for a few minutes

This is what I've always done when starting off a new (internal) sponge filter. But I leave the new sponges to soak for several days. It's very apparent that very little mulm seems to attach to the sponge at first. The new sponges seem to repel real wetting, bit like a new towel is no use to dry yourself with.