Unfortunately I'm not very familiar with the area West of Naciemento, but it's one of the more remote parts of California and I'm not sure you'll find many people who are. Just from the Google Maps Satellite view it looks like the Heart Castle Rd route between the Castle and Burnett Creek Road is unpaved, and possibly trail-like. Hearst Castle Road between the visitor center and the Castle, I do know, is closed to non-official vehicles. I'd imagine the back side is similarly closed off.
Fort Hunter Liggett is in a small valley, so it's easy travelling. But travelling West from there is pretty rough terrain, crossing the Coastal Range of mountains. It may be a fun drive trying to ply your way through the area, but I'd go with a backup plan that took me around Lake Naciemento on Interstate Road, through "Adelaida" whatever that is (I was there a couple years ago, I don't remember anything other than wineries there), and then cutting over on Klau Mine Road / Cypress Mountain Drive to Santa Rosa Creek Road, which will take up through Cambria.
Any way you work it, I'm jealous of your trip, because it sounds like fun and I love being out in remote areas like that.
If I had any other unwarranted advice to offer, it'd be to bring a GPS, and to get started early in the day. This is an excellent time of year to be travelling here (despite our recent out-of-season storms, they'll hopefully be gone soon) since the days are starting so early and lasting so late into the night.
