I think it depends on how the individual cables are shielded. If you can fin the Beolink Handbook (available for paying members on Beoworld In the Biotech section) you can see the cable drawings with pinouts. For the RJ45 Powerlink cables the wire collars are the same as n a network cable, so I will assume it can be used. I did use I Believe CAT6A network cables to make some short RJ45 to 8 Pin DIN adapters at some point, and they work fine. But might be different for longer cable runs.

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(On the extra note the transformer on my beomaster is really loud, is this common? Just tightening or adding some cushions could fix it?)

Danish Soundparts (a Beoworld Sponsor) do sell some new support rubber bushings that may help alleviate the problem:   I replaced the bushings on my BC9500; it still hums but to be honest I cannot tell if the humming is reduced!  The old bushings were very brittle, however, so new soft rubber should theoretically improve the damping.  You could also look at re-positioning the Beomaster away from any shelving that may resonate and amplify the hum.

The "Buzz-Buzz"ing you hear during during song and system changes is most likely the Datalink messages being sent by the BM to update the Penta displays.  You'd be able to confirm this if you disconnect the Datalink wires in the terminals and the buzzing stops. Just strategizing - a potential inexpensive fix is to run data in a separate 2-wire shielded cable alongside the Powerlink cable, and reconnecting them at the two Din-8 fixtures, or by using the separate 4-pin speaker connections to just run data from the BM to the Penta, and keep the Powerlink for just audio sigmal. I have not tried either method, so I can't speak to how well either method will work, including whether there might be other modifications needed to the Pentas to use method two. Good luck! Just an afterthought - there is probably a straightforward way to have DL signal broadcast wirelessly around the house from the BM via a DIY contraption - it's a very low baud rate.  A real Rube Goldberg solution.