Yes, actually on another forum a guy from Norway posted this message:
I just received my US T-Mobile activation kit in the mail from ebay (24/2/2015). I live in Norway and managed to activate it over the internet, pay for a PAYG data package (100mins, 5GB) using my Norwegian credit card. I have now started receiving SMS's on the phone, so now I have registered on the tmobile site and have my balances to hand etc... I even tried phoning my US number from my home phone and it rings! So all I can say is WOW! I fully activated my SIM card, paid for my data, all remotely,
So, I asked him:
So your verification SMS from T-Mobile US was forwarded on the Norwegian roaming network? Because, T-Mobile US used not to send it abroad. This may be changed now and can be a great help for people trying to activate before arrival. Was your activation process any different from the US? E.g. as described in the booklet?
And he replied:
"
This is how I did it:
1. Got activation pack and sim in mail from ebay.
2. Googled t-mobile and activation and went straight to tmobile activation website
3. Filled in activation number, IMEI etc. Phone activated almost straight away and showed my new phone number on the website. No SMS or anything at this point.
4. Then was presented with a top up page. Chose the $30 deal and proceeded to pay with my Norwegian credit card. Still no SMS. Phone was saying it was not registered when I tried to dial a number.
5. Tried to register on t-mobiles main site, in order to get the balance/topup page. It said it sent a verification sms, but i hadnt received anything at this point. I was beginning to think I was out of luck.
6. Three or four minutes passed and suddenly I received a flood of welcome SMS's as well as the code to enter on T-Mobiles main site. I guess the topup I did kicked in and the phone started working.
I didnt really read the booklet much, but I just followed all the instructions on the activation page. All worked fine. I didnt expect it to, as I thought I would need to be on US soil. Note, that Norway does not have T-Mobile so it was pure roaming. Perhaps the checks are not so stringent coming from Norway??
But it definitely works fine. I tried phoning my phone from my landline and it rang. I then decided to pull the sim card. I will pop it back in once in the US.
Thats a first for me!! Registering a US sim card in my home country 
) Kudos to T Mobile! It means I get internet access straight away and don't need to screw around with finding a mobile shop when I arrive.
So, activation on a roaming network outside of the US may finally work. This is new. But it needs to be confirmend by other users on other roaming networks too. So going to the US you might give it a try and give us your feedback