I wouldn't worry about it so much. You have no reason to be concerned about loyalty to an employer, much less a potential employer. I would recommend against disclosing that you only plan to take the job if your side project fails to take off.
But let's put this into perspective: one of my employers got very upset when candidates had already accepted a competing offer and still went through an interview loop. Your situation is almost the same, but it's about halfway closer to you being in a band that would like to go on tour if you manage to snag a record deal (which, I can say, a number of software/web startups are totally ok with; they want you as you can provide value to them).
I will sometimes disclose that I have side projects, because 1) it usually gets me street cred with the types of companies I prefer to work with and 2) I prefer to work with companies that don't mind that I have entrepreneurial aspirations and are ok with me working on some unrelated, non-competing project outside of working hours.
The tacit acknowledgment that you'd leave if it started making more money than you could pull in from your day job is more than enough. You can leave that completely unstated.
However, a number of companies are not ok with you working on side projects in technology, because they want you to focus all your cognitive energy on their business problems; that's why you end up salaried instead of hourly. But many new-school startups want people that want to solve their problem, then go home and geek out on something else, because there's value when people go explore areas outside of their day job's narrow scope. I frequently learn something outside of work comes in handy months later in my primary gig.
It is often worth asking about company policy on side projects in the abstract, but don't overemphasize it.
If you are presented with a formal offer letter, you may wish to bring up your dilemma, and tell them you need to give some thought to whether you want to take their offer, or to focus all your energy on your project. This may lead to a discussion of maybe a staggered start (3 days a week or something), a more aggressive sales pitch on their part, or nothing, but should give you a few nights to make a decision on whether to proceed or not, and may open the door to a later start date if all the stars align.
Under the circumstances, however, considering you still have a few months to go, I think your timing is a little odd. If I were working on a bootstrapped startup, I wouldn't start doing interviews unless I was worried about money soon (<2 months). Most companies want someone yesterday, though start date is always a matter that can be negotiated.