I know of at least a few people who matched to neuro fellowships, yet didn't know until internship that neuro was in the cards for them. While getting strong experiences (in addition to strong generalist training) at the grad school level is helpful, it isn't necessary. The Houston guidelines essentially say that your internship neuro intensity level should be reflective of your remaining needs as a trainee--if you have solid neuro grad school experience, then your internship can be fairly general (although you'll want at least some neuro work). Conversely, if you had little to no neuro experience in grad school, you'll want to spend 50+% of your time in neuro-related activities.
I honestly don't know of anyone who had limited neuro experience in grad school AND limited neuro experience on internship, and who then subsequently matched to a solid neuro fellowship. However, there are fellowships in the APPCN match that go unfilled each year (only a handful, but still), so I'm sure it's possible. I'd imagine it would make for a VERY intense postdoc experience, though. Neuro fellowships already tend to fall on the higher end of that intensity spectrum, and when you combine that with having to play catch-up, it'd be rough.
If that's what ends up happening, be SURE that your fellowship will allow you to have formal didactic experiences in all necessary areas (e.g., neuroanatomy, general neuropsych assessment principles). Otherwise, board certification could be even tougher to secure than the postdoc.