The only way you can do this properly is by adding another fixed layer in- so if you screw in cement boarding first through the render into the masonry, that can then take the brick slips.
If you're struggling cleaning up brick, the only real thing that can work is 'brick acid' (muriatic acid), otherwise try using brick slips to clad it and keep the effect (more cost there of course)- you can tile directly onto masonry provided it's stable.