The simpler design is probably to just fill a pool of entropy from urandom in user-space and give out numbers from that pool, thus amortizing the syscall overhe

The simpler design is probably to just fill a pool of entropy from urandom in user-space and give out numbers from that pool, thus amortizing the syscall overhead. Even that would be not-quite-as-good-as-urandom (still more that could go wrong), but it's probably as close as you can easily get.