You can actually obtain meteorite powder by playing arena, but not if you win 9 games (according to my personal experience) :
I played 57 runs of Arena since Homecoming (according to the contracts).
On those 57 runs, I won 6 times (9 wins). Each time, I received 1 non-premium gold card, 1 keg, around 300-400 ore and around 90 scraps, but never meteorite powder.
I noticed that you can obtain MP if you have between 4-5 to 8 victories (maybe also for less games but I'm not sure). The amount received increases with the number of victories. One time, I obtained 200 MP for 8 victories, and in my last run (today), I received 120 MP also for 8 victories. But it's not always the case. Several times, I won 7 or 8 victories but obtain no MP.
Personally, I prioritize MP (I currently have more than 400k scraps and 18k MP, and I already have the full premium collection; I forgot to convert my excess of scraps into MP before the change of the economy system) so I'm not so frustrated if I can't have 9 victories but only 7 or 8, if I receive MP of course !
So currently, for me playing Arena is the best way to win free MP, even if it's random.