There are a couple of ways you could do this, depending on what you may have on hand.

1. If you have converted some locos from 6080/6090x decoders to later model decoders then the decoders you removed could be used to switch the lighting. These decoders will use MM protocol only, but that should not be a problem.

2. ESU make a function only decoder for doing this sort of work. I don't know off hand if it does mfx, if you want them to self register.

3. One of the members of this forum uses TAMS decoders for this purpose. Again I think it is a function only decoder, I don't know if it does mfx.

Ciao
In addition to the good suggestions from Kiwiallan, you can also use an mLD3/mSD3 decoder e.g. 60972

they can be programmed to function as pure function decoders and they can then handle up to 1.1 amps on aux 5/6, ref the manual. You must set the motor type to 0, aux 5/6. (The green/blue wires then become aux 5/6).
note that you can also use the "normal" aux out in combination with a relay if the lights draw more power than the decoder can handle on the aux.

the mLd3 programming can be done with a CS3, either directly or with the mDecoderTool software through the CS3.

an old 60960 can also be used, but personally, I don't recommend that unless you have one laying around or you can get one extremely cheap.