An effective website must meet both user and business requirements, right? In the case where a business simply needs to gather qualified leads and the user is simply trying to learn more about the business, how can I create a true data backed persona that will inform my design decisions?
Ethnographic or contextual research would be ideal, but what is there to observe in this case? There is no tangible product and interaction is limited to main navigation and perhaps a contact form.
Is the concept of a persona not applicable to this case?
I'm not sure I understand your question. If the question is: "I'm developing a product page, but I don't have a product yet. Should I use personas to drive the product page design?"
Then my answer would be absolutely. Even if you don't have a product, you should design your product and web site in order to captivate and address the questions of the customers you are trying to target.
You could argue that you want to develop a web page that is just for the purpose of creating lead generation, so the page could have a generic message and asks people for their emails. But wouldn't it be more effective if you designed the page in a way that I understand what you are building and made me interest in it? I think it would increase the likelihood of me giving you my information.
Personas allow you to focus on the customer segment you are targeting. Every decision should be made taking your customers into consideration (even if you are still trying to get customers). Instead of thinking about the customer that might buy your product, it helps to think about Suzzy that is a mother of 3 and would really be glad if she found that the product you're building is available at her favorite retail store.
The goal of your persona is not to observe users figuring out to navigate your prototype (that's usability testing). The goal is to create a rich description of a typical (but not too stereotypical) user, that can easily be communicated to the team, for everyone on the team to be able to empathize with your users.
If you want to use observations, you would want to observe pretty much anything they do - not just limited to how they would be affected by your offerings - in order to build the needed context to truly understand who you are designing for.
Then, once you have the personas in place, you might go about creating some hypotheses as to how to improve your design and start testing those hypotheses, but this part comes after the personas (although I suggest that you continuously improve your personas based on new findings).