Using LittleProxy with Selenium is very strightforward. All you need to do is start a LittleProxy instance and provide its address to a Selenium Proxy object. In fact, that is what the latest BrowserMob Proxy does. See BMP's
ClientUtil.createSeleniumProxy() for an example of how to create a Selenium Proxy object from an InetSocketAddress.
HAR capture is more complicated. LittleProxy is only a proxy and so doesn't support HAR capture, but can be extended using filters, etc. to do so. Once again, this is exactly what the latest BMP does with LittleProxy. Have a look at
HarCaptureFilter class to see how BMP does it, and perhaps adjust to fit your needs.