Drive carefully at the creek crossings, but I suspect the gates will be locked unless the creek crossings are safe for virtually any vehicle. The gates were locked on two out of my three trips to the trailhead.
In the case that the gates are locked on Del Venturi Road in times of high water, you can still get to the trailhead by following Mission Springs Road around to Milpitas Road, thereby rejoining the normal route beyond the 2 gated stream crossings. Instead of turning left on Del Venturi, continue straight. After a brief moment, you will notice an intersection that presents three choices. Do not veer left unless you want to check out Mission San Antonio de Padua or use the port-a-potties there. Do not follow the right sweeping curve that leads you into the military base's resdential area. Instead, choose the middle of the three choices by continuing straight onto the narrow road with faint double yellow lines. The road soon becomes a high quality, smooth dirt road, passable for all types of vehicles. Ignore all possible intersections along the wide dirt road and continue straight. After several miles, the dirt road will end at the normal Milpitas Road paved route. If you look to the left, you will see the locked gate at the second Del Venturi/Milpitas Road stream crossing. You will turn right onto the paved Milpitas road route and continue to the trailhead.
I think the cop warning mainly applies to Memorial Day weekend and other peak visiting times to the Jolon Road area or the base(possibly Easter/Spring Break and three day weekends). I am not discouraging anyone from playing it safe when visiting at quieter times, but in my experience one is only likely to encounter law enforcement in the short distance between the entrance to Fort Hunter Ligget and Mission San Antonio de Padua, in which case it would be military police strictly enforcing speeding and other traffic laws, such as not talking on a cell phone while driving. In one trip to the mission and three trips beyond the mission to the trailhead (three trips late April and one early February), I have never seen any type of law enforcement outside of that small stretch of road near the residential heart of the base, other than CHP on HWY 101.
Be sure to have your driver's license, vehicle registration and proof of insurance as they will be checked at the entrance to Fort Hunter Liggett. Passengers over 16 years of age should also have a photo ID. Tell the guard you are going to "Indians" and you will be given a vistior pass to put on your dashboard with your destination indicated. Observe all regulations posted on road signs.
On the way up from Santa Lucia Memorial campground we met a group of hikers at the trail junction from Last Chance campground and their report was that their section of the trail was nearly impassable due to the overgrowth (they reported crawling through some sections). They didn't make it to the summit and instead of battling the brush again chose to hike down to Memorial campground and take the closed road back to Arroyo Seco campground where their vehicles were.
The Jolon gate was not manned when I visited. However, Mission Road appears to have been permanently closed to the public between Nacimiento-Fergusson and Del Venturi. A guard directed me onto Nacimiento-Fergusson and around the closed area to the Mission / Del Venturi intersection.
The trail from Memorial continues past the old tractor to the broad drainage to the east. It rejoins the route shown on the topo at the 4,150 saddle.