You can get a close to what you are suggesting by setting the end date of the full century to your actual event date in August. So if you only have 8 weeks left until your event you’ll skip the first 4 of 12 weeks of the century plan.
Alternatively, if you do have a full 12 weeks available between your metric century and century ride but are looking to start the century plan at a higher intensity, check out the 100 mile gravel grinder plan. From what I can tell it starts at a higher volume than the regular century plan.
I don’t know when the 2nd event is. They’re still working logistics but it is usually in August. My metric century is June 13th but the event could be canceled with COVID.
My limiter now is time and my willingness to ride over 3 hours indoors. I live in the northeastern US and we still have frigid temps. I may kick off a gravel plan after my metric plan. I would enjoy the long rode in nice weather a lot more.