As a client, why would I want to hire a team that I know is focused on something else? I don't want my project to be treated as a means to an end for the startup to achieve their dream.
How are you going to address this kind of apprehension?
Everyone is focused on something else. If not a side project or a startup, an SO or a hobby.
By definition, if you are paying someone money, they are focused on something else.
Also, we've seen larger startups (4 or more people) that need to bootstrap tend to only give up a portion of their team to take a contracting gig while the rest of the team focuses on the startup. Not unlike raising a round of funding.
Full disclosure, I'm doing something similar right now, and the employer we are working for is truly satisfied with what we're doing.
If you have something that needs to be built and you want a long term commitment you should simply hire experts as full time employees.