They have part-timers who were once full timers but typically you can't start off part time. Closest thing is the summer associates program for visiting PhD students.
Ps. I had an office overlooking the Santa Monica mountains and got to spend time deeply immersing into interesting problems. Now at current tech co I'm packed into an open office plan elbows to elbows and don't immerse much past any problem that can't be solved in a week or too. I sometimes hear Gob saying in my head, "I've made a huge mistake..."
A couple years ago I was considering a move to Santa Monica and while my obvious employment would have been some form of software development further from the beach, I thought it might be cool to find my way into RAND in some capacity.
Do they have part-time researchers there, or maybe researchers-in-residence for sabbatical-length periods?