Is there any other MOSFET that can be used as the external switch with an LTC4279 PoE PSE controller? The data sheet strongly pushes the Nexperia PSMN075-100MSE and PSMN040-100MSE as the only choices, and they're out of stock worldwide for 6 to 16 months, and the parameters that lead to these choices aren't explained. I'd love to use something generic like an IRF540NS as the switch, if I could. They're a little large but they're cheap and plentiful, have an even lower Rds(on) than the Nexperia parts, and seem like far more than enough to safely switch a Type 2 PoE load (half an amp), unless the PoE negotiation depends on very specific magic. But the data sheet makes it very clear that I'm not qualified to choose.
Please contact Nexperia directly for availability of these parts.
For second source MOSFETs, consider the Fairchild IRFM120A, FDT3612, FDMC3612, FDMC86102 . These have been used in IEEE802.3at Type 2 applications with previous generation ADI/LTC PSE controllers that have similar inrush and foldback requirements as the LTC4279.
Rds(on) is certainly important for efficiency and low heat dissipation in normal operation when the MOSFET is on and the port is drawing maximum current. But the PSE MOSFET selection goes beyond choosing a low Rds(on) device. The MOSFET must survive Hot Swap conditions with harsh current limits for a set time during inrush and fault conditions. The MOSFET must have a safe operating area (SOA) above these conditions over the full system supply voltage and temperature ranges. The MOSFETs recommended by ADI/LTC have been tested and field proven to have adequate SOA for a robust solution.