There are fixed relectors (we've had Lensref model). Practical sailor reviewed these over the year.

The Echomaster is a hoistable, inexpensive and very effective device that seems to me difficult to permanently mount either practically and/or visually. :^)

There are transponders which are powered, but the models I've seen are pretty expensive. (Like $1200 or more, materials only.) There are some units that have been around which broadcast only in the x-band (not s-band) radar frequencies. For example, the "Sea-me" units... http://www.sea-me.co.uk/shop.html

I'm not sure how these are affected while you're using your own radar. We have a "passive" alarm reciever plus our radar. If you turned the radar unit on, you had to turn-down the sensitivity on the passive receiver. We gave the passive reciever away after a few years. It worked well, but if we were going to be offshore, we run the radar full time. (It might be different for others, but for our sailing routes and practices, we either have the radar on or we're not worried about needing to see or be seen. We use so much power that it just doesn't make that much difference ;^) )