Great brands need to stay on top of visual. No matter the size, it's easy to put something together that doesn't quite carry the brand how it should. But it's ok.
Contracting at an hourly/daily rate is a pretty popular way to keep the brand heart beating. I've worked with brands of all sizes on print assets for conferences/events, direct mailers, social media content or simply cleaning up presentation slides.
This relationship works quite flexibly. Whatever or whenever you need creative, just flash the bat signal.