I'm afraid it would involve some pretty major software changes to Espruino to get it to work. On the hardware side, the 5th pin on Micro USB isn't brought out anywhere, and to do it properly you need some power switching arrangement as well to supply power to the client device.
Basically doing it that way would be really difficult.
However, this has been asked a few times before, and there's a chip that will handle USB Host for you called the MAX3420/MAX3421: http://forum.espruino.com/conversations/284427/
It's available on a board with everything you need from a few places. A quick search on eBay for arduino usb host shows you can get them for under $5 (although I'm not 100% certain those use the MAX3421) - so not much more expensive than a USB OTG cable.
arduino usb host
There's no code for it for Espruino as far as I know, but the example code looks like it would be trivial to port over and I'd be happy to help if you have any questions about it.
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