What makes you think he'd have to call on Mab for more juice? Why wouldn't he just call on Winter?
Or, in other words, I would absolutely allow, with sufficient justification, a character to purchase capability with Sponsored Magic without having an embodied, personified, or even known sponsor.
edit: for clarification, I would not place any mechanical restrictions on this power, but the Compels would likely be different from those against a character dealing directly with a known personified entity acting as a sponsor