I know what you mean, a bayonet to female filter thread adapter. I haven't come across one either.

There are several alternatives to reversing a lens, which only gives you one (high) magnification. Extension tubes or, better still, a set of bellows. Most bellows use M42 for both ends, you will need a bayonet to M42 at the camera end, there are 100's of high quality M42 lenses available cheaply for the lens end. Now you have a variable magnification.

The old B.P.M. bellows are often available on eBay and there are adaptors to fit both ends of the bellows, all the major bayonets (both male and female) and screw threads are available. Even focus rails to fine tune focus. A whole site dedicated to them here.

http://www.macrobellows.com/

You'll often find them on eBay for a lot less than this site charges for them.

Forget LED macro rings, they just give a constant low power light, which means you are having to use Aperture mode and longish shutter times, the idea of flash is that the very short flash duration (faster than 1/1000th of a second) 'freezes' any camera movement, eliminates any camera shake, and even allows hand holding when 'bug hunting'. You can use any cheap manual flash for this not just a ring-flash, though with manual flash it's manual on the camera too, but once you have the flash power set for the magnification and small aperture to get the correct exposure it doesn't alter and you can ignore getting the exposure right, it always will be, the small depth of field means the flash subject distance is always the same.

Chris