Your GM can rule otherwise, but as the rules are written, no. A Whirlwind Attack is not a series of Half Actions, it is a single Half Action that causes multiple attacks, so there is no 'in the middle' to take that Free Action. Your example is not interrupting an action, but taking Free Actions (leap up, yell, quick draw) before/after the regular action of attacking.

Even you could use the leap mid-Whirlwind/post-Whirlwind, any additional enemies who enter your melee reach were not in reach when you took the Whirlwind Attack action, so they wouldn't have been eligible targets.