The knock sensor won't adjust fuelling, it will adjust timing. But the two most dangerous side effects of running lean are knock and high EGT.

Most ecu's can have a prog output to enable a warning LED, The warning light can be used to warn about a number of dangerous conditions such as water temp, AFR, EGT, MAT, KNOCK etc.

Having one warning light imo is more noticable than a whole host of gauges, when the light comes on, pit and then look at the gauges to work out whats up.