Here's mine...

Looks rather silly, but is historically accurate for Mirambo's Ruga Ruga. Wherever he was on the battlefield he was accompanied by an umbrella. As a young man he would fight alongside his men, and his presence would be indicated to the troops by the brolly. Because he always gave a greater share of the booty to those he considered the bravest and best, knowing he was nearby encouraged the men into more acts of reckless bravery in the hope of catching his eye, so it did function in much the same way as a standard.
With other peoples it's a job to know what to do. Many of them didn't really have the concept of a standard bearer. Some peoples (like the Ngoni sometimes) would have their chief present at a battle, sat in a prominent but out of harm's way spot on a stool at the back. Not really taking a leadership role (there would be generals for that), but more of a ceremonial one. I suppose you could give such a figure whatever powers a standard has in your rules, though you would want to discourage them from actually taking part in the fighting (by making them rubbish at it, for instance).

Personally that is my plan. My Ngoni king and his cheerleading squad of dancing girls. He'll have "army standard bearer" powers, but limited movement and may not initiate combat. Instruments and witchdoctors I'm keeping separate, with their own distinct rules to add flavour.