No "Custom Properties" on ascii fbx exporter

There’s no ‘custom properties’ field on the ascii FBX exporter but this is present in the binary version. I’d like to use custom fields generated by a plugin I’ve written, with a mind to getting Unity to then parse them.
Has this been deliberately omitted (maybe due to due to format restrictions in 6.1 ascii FBX?) or is this just an oversight?
I’d just like to clear that up before I start hacking at the python files.