That was hard to answer! But here we go. First let’s clear the ground:
1. denying yourself needn’t = lack of ego. Someone can live ostentatiously for others & be a great egoist. Likewise, the Buddha spent years eating leaves and found it didn’t help him get away from the ego.
2. You can mislay your ego + not get any better. Falling in love – suddenly the other person – rather than your ego, is the centre of the universe. It might help you be better, but it might equally make you awful. But I don’t think that’s ‘self-indulgence’ either.
With those in mind: I think lack of ego becomes self-indulgence when the different parts of you want different things, so that you can deny one part while feeding – indulging! – another part.
I think Henry does this in Henry + Cato. He’s denying one bit of himself (that wants the hall & the money) but feeding another bit (that wants to spite his relatives).
That’s my answer!
Much Platonic love, Sam