This is a WIP with only a couple hours put into the code, but I think it works, give it a shot, send some pull requests if you run into issues.
npm install react-router-relative-links react-router-apply-middleware react-router-route-props-context
import applyMiddleware from 'react-router-apply-middleware'
import routePropsContext from 'react-router-route-props-context'
import { RelativeLink } from 'react-router-relative-links'
// use it like other router middleware
<Router render={applyMiddleware(routePropsContext())}/>
// now you can use `RelativeLink` anywhere \o/
<RelativeLink to="../">Up</RelativeLink>
<RelativeLink to="./down">Down</RelativeLink>
<RelativeLink to="../sideways">Sideways</RelativeLink>
<RelativeLink to={{ pathname: 'foo', query: { bar: 'baz' } }}>location descriptors</RelativeLink>
<RelativeLink to={{ query: { bar: 'baz' } }}>just the query</RelativeLink>