In Unix land the way I would resolve this is move the RenderCache directory to somewhere on the SSD and create a softlink in the orginal parent directory called RenderCache. The app will still think the RenderCache is still in its old location when in fact its on the SSD.

Windows has a similar junction/softlink system. I've never had to use it so can't give you any practical advice but if your interested checkout articles on MKLINK and Sysinternal's junction.