I have tried my hand at some "ultra-macro" with a cambo 8x10 monorail (inexpensive) and a little Nikon 120 AMED, the lens that became very inexpensive recently. As I recall, when using a diopter with that, 5:1 was no issue at all and I simply found it too annoying to focus after that.
If I wanted to push further my advice would be to get that same camera, a long piece of aluminum extrusion to subtitute for the rail, an extra rear standard so that you can attach another segment of bellows, and that'd do it. Something like a cambo has the stability for ultra macro. The major issues at high mag are stability and well-controlled focusing. The good news is that the shutter impulse and such won't matter because you will be taking looooong exposures when you go appreciably past 1:1. You can use your lighting control to control the exposure- you don't need a shutter at all.
But I would foremost think very carefully about the overall stability of your platform. People use heavy macro rails for a reason. A long-axis optical rig has real vibrational modes to worry about. In my profession we use inconel for this reason! Fortunately you won't have to go to that extreme, an old cambo would do nicely!