I don't see any reason you couldn't use the sponsored magic mechanic for other things. I can see issues with that specific adaptation of sponsored magic. Like one of the downsides with sponsored magic is that it can be restricted or withdrawn entirely if it doesn't fit the sponsor's agenda or if the sponsor simply desires it. That's not well represented in a personal sponsored magic (you will always be within your own agenda). Another issue is that of debt. I can see where you are going with the compels to return debt (and I like it), but where is the fate point coming from? How are they gaining that additional burst of power?
And I think all those things can work with a purely internalized source.
If you accrue a lot of debt, then that means you are, in some manner, acting against your nature. If you had then you'd be repaying that debt. Eventually continuing acting against your nature while drawing heavily on your inner power (racking up debt to empower your spells) will weaken you until you just can't cast anything at all. It's like an introvert going to big parties day after day and being social. He'll get drained doing that and need to recover from all that annoying socialization. In the case of internally sponsored magic, instead of needing a nap and a few days alone to recover, you'll need to take some time and do whatever that particular aspect of your nature needs to make you feel better.
Also, you can act against your own agenda in various ways. Remember, Sponsored Magic is kind of like an aspect. No one aspect, even the High Concept, fully represents someone. Any PC is going to face thing where parts of their nature come in conflict or they just come in conflict with the best way to get things done.
We can easily imagine an example of this. Consider the 3rd book, in which Michael goes to the Masquerade party. He's deeply conflicted about that, because his gut is telling him to destroy all the vampires there or die trying. However, that's not the best way for them to get things done that they need to, and Michael understands that (though it makes himself unhappy). Imagine now a different Michael with Soulfire as an internal source of power.
The part of him that represents really wants him to lay waste to the evil all around him and will cause a compel just like an aspect would. This might well tie in with one of his aspects, leading to what is essentially a scenario where multiple aspects are compelled (and hence he'd have to spend TWO fate points to not do something kind of crazy).
As for where this power is coming from, one could say the same for any wizard tapping an aspect. Where do fate points come from?
Or, one could look at Harry and Soulfire and see that Harry's power there isn't coming from an outside source, but from him literally tossing his soul at someone.
Or if the character is a demigod, then one could say it is his divine nature that the character is tapping into somehow. Or they could have some sort of prophesy about them that foretells of them having great power due to some sort of mystic alignment at their birth (or whatever). There are lots of ways to propose where this inner reserve of power comes from.
