Here's what I did.

import { DefaultUrlSerializer, UrlTree } from '@angular/router';

export class LowerCaseUrlSerializer extends DefaultUrlSerializer {
    parse(url: string): UrlTree {
        // Optional Step: Do some stuff with the url if needed.

        // If you lower it in the optional step 
        // you don't need to use "toLowerCase" 
        // when you pass it down to the next function
        return super.parse(url.toLowerCase()); 
    }
}

And

@NgModule({
    imports: [
      ...
    ],
    declarations: [AppComponent],
    providers: [
        {
            provide: UrlSerializer,
            useClass: LowerCaseUrlSerializer
        }
    ],
    bootstrap: [AppComponent]
})

A little fix to Timothy's answer, which doesn't change the case of matched parameter names and values:

import {Route, UrlSegment, UrlSegmentGroup} from '@angular/router';

export function caseInsensitiveMatcher(url: string) {
  return function(
    segments: UrlSegment[],
    segmentGroup: UrlSegmentGroup,
    route: Route
  ) {
    const matchSegments = url.split('/');
    if (
      matchSegments.length > segments.length ||
      (matchSegments.length !== segments.length && route.pathMatch === 'full')
    ) {
      return null;
    }

    const consumed: UrlSegment[] = [];
    const posParams: {[name: string]: UrlSegment} = {};
    for (let index = 0; index < matchSegments.length; ++index) {
      const segment = segments[index].toString().toLowerCase();
      const matchSegment = matchSegments[index];

      if (matchSegment.startsWith(':')) {
        posParams[matchSegment.slice(1)] = segments[index];
        consumed.push(segments[index]);
      } else if (segment.toLowerCase() === matchSegment.toLowerCase()) {
        consumed.push(segments[index]);
      } else {
        return null;
      }
    }

    return { consumed, posParams };
  };
}

Edit: beside the problem explained above, there is another subtle bug which is resolved now. The for loop should iterate over matchSegments instead of segments.

update

This didn't make it into the new router yet

original

Regex matchers were introduced recently. This might help for your use case.

See https://github.com/angular/angular/pull/7332/files

And this Plunker from Brandon Roberts

@RouteConfig([
  { name : 'Test',
    //path : '/test',
    regex: '^(.+)/(.+)$', 
    serializer: (params) => new GeneratedUrl(`/${params.a}/${params.b}`, {c: params.c}),
    component: Test
//   useAsDefault: true
  }
])