Only if they were willing to pay for it themselves. There will be the odd enthusiast who does, (I can think of a few in this category, including one who owns a couple of old buses!) but neither uses it for driving commercial bus driving. TOCs would likely take a dim view of a driver moonlighting as a bus driver (or any other job) for fatigue reasons, not to say it wouldn’t be possible to work around it for those who were really keen; (
@notadriver ) may be able to give more insight.
If a train driver wanted to earn extra money they’d also be financially far better off doing an overtime shift at the day job, rather than driving rail replacement buses.
That’s the handbrake, right?