Only a portion of mstatus is shown in the manual because that particular section is dedicated to interrupt handling. If you check the Privileged Spec most of the upper bits aren’t supported on the FE310 because it doesn’t have MMU, Supervisor Mode or floating point support. Unimplemented bits will read 0x0.
The FE310 does support User Mode, and I do see Modify Privilege (MPRV at bit 17) programmable. Timout Wait (TW at bit 21) isn’t programmable which indicates it’s not supported. In fact when a WFI is executed from User Mode I see an illegal exception taken.