Highly unlikely, that would undermine their business model. For the same reason they don't offer pay as you go plan. Vlad explicitly stated somewhere that they are able to make money only because many people do not use their plans to the fullest.
There will certainly be a level where this is viable, if not sensible. Rather than having “casual” users cover the cost of “extreme” users, letting them specify an API key will likely be beneficial (albeit for a small number of users).
I mean folks would still be paying for an account, just not at the ultimate level. And since they offload the actual LLM processing to a third party, not much overhead cost there.
I imagine the intersect between paid Kagi users and paid LLM users is pretty high, and many people probably don't want to double-dip on LLM spending.