Is it possible to use 50ohm cables for these short runs?
I have those available from my wireless system.
The fundamental frequency for a 64 channel MADI stream at 48 kHz sampling rate is somewhere around 75 MHz. That has a wavelength (λ) of about 4m. If you follow the "rule" that transmission line principles should be followed once you're going farther than 1/8λ, then you should be using 75Ω cable, even for a relatively short run of only 5m.
I'm making some other assumptions here, including that the velocity of propagation in the wire is 100%, which it never really is. A 5m 50Ω cable might
work, but I would not count on it for anything mission critical (anything other than testing in the shop when you're the only one listening).
If you know anybody who works in television, you could probably get your hands on a short cable very easily. That's what I do at my day job (TV and radio), and we have stashes of short jumpers all over the place, and the tools and supplies to make up any length we need on short notice. Even the old Belden 8281 cable ("Belden Blue") with the 50Ω connectors would work for what you want to do, although it has been culled from our plant since we went DTV.
Going the other direction, there are length limitations with copper cable (75Ω HD-SDI coax). I have a run of something more than 100m from one studio to the loading dock where the mobile is parked, and I can't get a MADI stream over it with any reliability. Fibre is fine of course, and AES (stereo) will work on the trunks, as will NTSC video sync, but we have to convert to fibre to get an HD-SDI signal down there.