> current employees have access to a sell-to-cover option, while former employees are required to prepay in cash
This is common. Cashless sales are a benefit. We strictly regulate who can and cannot be provided employee benefits in America. Your former employer would risk extending a securities-based loan to you. It can be done. Your former employer decided not to extend the privilege.
[1] Find the IPO pitch materials and see if the bankers pitch anti-dilution post IPO.