vluint64_t next_usercb_time = 0;

...

// Initial user callback at time 0
next_usercb_time = user_cb(top);

...

while (!Verilated::gotFinish())  {

    ...

    main_time = VerilatedVpi::cbNextDeadline();

    // Don't skip past the next user callback time
    if (next_usercb_time < main_time) {
        main_time = next_usercb_time;
    }

    VerilatedVpi::callCbs(cbNextSimTime);

    // Call user callback for clock toggle
    // Save next time to call it
    if (main_time == next_usercb_time) {
        next_usercb_time = user_cb(top);
    }

    VerilatedVpi::callTimedCbs();
}