I don’t think that they couldn’t do it skill-wise, but there are scenarios where a BNR might have to make a choice between competing one or two additional horses, vs having adequate time and endurance to warm up all of the horses properly.
If five owners want their horse to be ridden at the same trials, or the rider/seller wants this competition on a horse’s record to increase their sales price, and the rider/seller feels easily replaceable, and the rider/seller has recently carried and birthed an entire human being out of their body -maybe someone could feel tempted (or even professionally obligated) to break rules.
I don’t even know…if another rider were identified on a horse in warmup, what is the consequence? Is the punishment as low stakes as that specific horse being disqualified from that specific phase? Would the competitor on record be disqualified for the rest of the event and not be able to compete their other horses? Additional sanctions?
I don’t think we all need surgically installed microchip implants so volunteers can scan us. A volunteer could check ID (cue BNRs hiring grooms who could pass as their twin), put a stamp on the rider’s hand that matches a stamp on their number etc. when they check in at the event, etc. It’s not realistic to expect an untrained volunteer to try to recognize every rider and horse by memory and confront them.