Oops. Stuff did break.

I said that my quick test showed things running after my dreamhost server move. I think something must have still been running on the old server unfortunately, as when I checked later I was getting server errors back from passenger. It didn't look like paths, which is the main thing that Dreamhost warn about, since the errors were being triggered from within the merb framework, so my custom gems were obviously loading. I figured it out by exporting the same GEM_PATH as in the config.ru then running:
bin/merb -V -e production -i
The error of which pointing to the loading of a shared library file for ParseTree. That flipped the brain-switch to on and I realised that I'd been moved from an x86 server to an x64 so the libraries would need a recompile. A quick:
bin/thor merb:gem:redeploy
And all was good.