Let's say the weather is 2,800 overcast, visibility 8 miles. ACME AIR checks in passing CEDES and requests the FMS visual. The clearance, "Descend via the track and altitudes of the FMS visual. Expect clearance for the visual passing JANYY."
Technically you're not on the procedure until you're cleared for the visual and fly it.
At SFO they may tell you to "join the FMS Bridge visual for routing only". But this isn't clearance to fly the FMS Bridge visual approach. You're simply flying a set of waypoints at that point (possibly in IMC) and provided all the usual IFR protections / separation by ATC. Nothing of concern here.
But you can't fly the visual approach procedure in IMC. ATC must confirm with the pilot if the airport environment is in sight before issuing the clearance. And the pilot cannot accept the visual approach unless he/she can execute it without going back into IMC.
Once you accept the visual approach, different rules apply, e.g., you're responsible for separation from the aircraft you're following, etc. However, there's no "hybrid" situation where you're somehow flying a visual procedure but in IMC with less protections than other IFR segments.