Sandline and Executive Outcomes come to mind, and there are probably a few former Selous Scouts around who might not be adverse to becoming the nucleous of a merc battalion.
But:
have you any thoughts on how the logistics would be handled?
The nearest seaport is in Mozambique and is 250 km from the Zim border and i know of no merc outfit that has a CV or LPH from which to launch a commando.
If Mozambique or South Africa would be willing to be a party to such an adventure, they would be better politically advised to do it themselves lest they be accused on cowtowing to a neocolonialist adventure.
Perhaps a trek across the Kalahari?
Sorry, but it is not going to happen.
The last time mercs were used was in Sierra Leone which is on the coast and also the mercs were hired by an African government. They were busily cutting a swath through the RUF insurgents when African and world opinion forced their removal