The predominant protocols/APIs offered by FX ECNs are FIX and ITCH. So you might want to look out for node.js implementations of these, e.g. nodefix.
Without that ``extra'' layer, the trading venue that comes closest to your (bulleted) requirements would be Currenex with their STP connectivity, a RESTful API based on FIXML. I put `extra' in quotes because it's now up to you to provide a neat internal framework that exposes node.js routines to your generator and FIXML to the Currenex API.
As for your additional constraints, I think they're a bit off-place. See it from their point of view: Any ECN with liquidity in the interbank league would put their systems at risk by offering such a service directly to just about anyone. And a broker connected to an ECN would somehow have to pass on the connection costs.
So your best bet might be dealers. They're much more likely to support your scenario (or at least might do in the future), for instance for stocks I've found Tradeking, a node.js API around FIXML (much like Currenex'). They have an FX marketplace too (broker-dealer by the looks of things), maybe you could ask them to open their API to that.