Yes, for domestic routes on widebodies with the 4-class configurations, the PS seats are sold as Comfort+ and thus available to Plat/Diamond elites at booking for free. (Similarly, the C+ seats are sold as preferred so available to Silver/Gold elites at booking for free also). Certainly for a daytime flight it hardly makes sense to pay to upgrade to F and the flat beds if you can confirm a PS seat (which is substantially similar to a domestic F seat), but the calculus may be different on a redeye.