This won't work for a P7 since its a poweline device, I guess, but the Power adapter on the P9 Plus at least is using standard 12v DC power connector used on security cameras etc - I mounted mine on the ceiling and made sure i got good quality 3 Amp extension cables and it all seems to be good.

Automat wrote

I have looked at my house and could possibly do with a 2 pack if the main unit -the one connected to the router via RJ45, can be hung on a wall. I have seen aftermarket wall mounters, so it shouldn't be a problem.

 

However, being a powerline, I guess that the original power cord should be used. I'd probably need it to be 4-5 m long. Is there a genuine tp-link power cord that long? Where could I purchase it if it was available?

FWIW I have been running three Deco P7 units for two weeks and did some wifi coverage measurements. It seems that the antennas in the units are optimized for horizontal emission/reception (i.e. they are transmitting stronger signal close to the disk plane, compared to the disk axis). So depending on what exactly do you need to cover, mounting on the wall might not be optimal.

For what concerns the powercord I can confirm it is on the short side of the powercords (even for TP-Link devices), probably <1,5 m. I guess it is partially because the same cord also carries the data. The low voltage connector is however USB-C type, so if you want to extend it, you would need USB-C extension cable. Whether it will work also for the data connection, only TP-LINK can tell.