I made an unreasonably hacky baby step.

const crypto = require('crypto')
const ssbKeys = require('ssb-keys')

let addr = ''
let keys = null

while (addr.startsWith('fc') === false) {
  keys = ssbKeys.generate()
  const { public } = keys

  addr = crypto
    .createHash('sha512')
    .update(public)
    .digest('hex')
    .slice(0, 32)
    .match(/.{1,4}/g)
    .join(':')
}

console.log(keys)
console.log(addr)

{ curve: 'ed25519',
  public: 'UoiIIO4uFUZMfE4svS0uY2ZZqyixAaafVbc+8nNq71U=.ed25519',
  private:
   'redacted',
  id: '@UoiIIO4uFUZMfE4svS0uY2ZZqyixAaafVbc+8nNq71U=.ed25519' }
fcdf:a047:9ab0:c779:1a9d:6846:0628:bba9