Is there a hidden way to fast charge an iPhone/iPad from a newer USB-C Mac?
No, a USB-C to lightning cable that is Apple certified will charge iOS devices at the fullest capacity the new MacBook Pro will provide to a USB and lightning connector. 

That same cable will fast charge from USB-C PD compatible chargers like the Apple ones.  

iOS doesn’t present as thunderbolt so you don’t get that amount of power over lightning connector. 

The technical details are available to any organization that participates in the MFI program - https://en.wikipedia.org/wiki/MFi_Program and the signaling between devices to decide to fast charge is somewhat sophisticated and possibly has an encryption / identity key (I haven't seen the specifications so that part is speculation).

There’s nothing hidden. Apple could decide to increase the charge since they control all the drivers and power management and SMC and lightning specifications. They just chose the 5w you are seeing.