Aware that German vehicles operating in North Africa were eventually given a coat of Gelbbraun over their original Dunkelgrau paint scheme, I was wondering whether there is any evidence that this would have been applied to the underside of the hull, under the track guards, and to the rear faces of the road wheels.

Well not all the kit sent to Libya was Dunkelgrau first, just the first batches of 5. Leichte Div.. Later shipments were factory painted in the Tropen scheme without any Dunkelgrau being applied first (for example, the Panzerjaeger with the 7.62cm Soviet gun on the Pz 38t chassis).

Having said that, there was no reason to overpaint the areas you mention as they would be invisible to an observer. The colour of the underside of any in service tank is largely academic as the belly panel is invariably streaky rust and dirt after not very long.