Steve:
I know the road into the park of which you speak. I have driven the road up to the turn back into the BBRSP and on into the park to the dead end.
Frankly I had never followed the line north until just now. On my maping software it does not distinguish between road types, but on my GPS unit the entire road system between 170 and 169 at Plata shows to be county roads. The split past Casa Piedra appears to loop back through BBRSP, but I know the road is blocked a few (many) miles past the ranch headquarters.
The road heading east from Casa Piedra past Elephant Mountain and on to 118 appears to be all public. The funny part is the roads only connect if you assume you can use a runway section that is just south of Needle Peak.
There is a road heading off 169 through Peridiz that appears to connect to 118 as well. There is a road at Peridiz on both sides of the railroad track but no clear connection across the tracks there.
Obviously, the people tracing the USGS quads and or aerial photographs got lazy at times. because I have found a fair number of errors on my treks around Texas. I have visited over 200 of the 254 counties in Texas and expect to hit the remainder this summer.
I spoke with a kid filling up a beat up quad at the store in Study Butte and he told be that there were tons of unmapped roads and trails in the mountains north of Terlingua. He said take the county road north out of Terlingua and you couldn't miss them. He also said there are miles of roads behind the housing development north of Study Butte on the east side between 118 and Terlingua Ranch.
As far as what I ride, I have a couple of true dirtbikes and no DS at the moment. I have contemplated plating my KTM 400, but I haven't done so yet. It would be great, but it doesn't have a cush drive, so I would have to be gentle with it on asphalt. It does have the big tank though.
If KTM ever brings back the 640 (rumored to be this year) I have hopes that a few folks will move up and I can pick up an older 640.
As far as when, not nearly often enough. I own my own architectural firm and I never seem to get projects close to home, so I am always on the road. I end next week in Fort Stockton (lunch on Friday) and if it wasn't for the fact that I have already promised to help build some fence, I would head south from there and check these roads out. Maybe it will rain here in the metromess and I can head that way anyway!

Now if I had a DS bike, I would plant that sucker the bed of my truck and take it with me ...