If you don't have reserves, borrowing from the Fed would be your very last resort. The first thing you would do is borrow short term from other banks to meet your reserve needs. This is what the Overnight Federal Funds rate represents -- the rate that banks use to lend to other banks short term.
AS a very last resort, you might borrow from the Fed, but banks will generally avoid this as much as possible -- The Fed often charges a higher than market rate to discourage banks from using the discount window too much. Furthermore, borrowing from the Fed's discount window comes with a lot of stigma, since it implies you are unable to get funding from the market.
The most recent Odd Lots podcast episode has a great primer on the Fed Discount Window
> I was assuming that without a central bank account, reserves are moot
There are tons of ways to get reserves without a central bank account -- issuing stock, selling CDs or Bonds, selling liquid assets, etc.
> This is because the central bank determines reserve requirements for the commercial banks, so a bank without a fed account is limited in how much money they can 'create.'
FWIW, in the US the reserve requirements are currently 0%. They're also 0 in a lot of other countries (e.g., Canada)