I was never disallowed to send friend requests to supporters in matches which I saw. For my teams, very few of T11 friends are added because we are facebook friends first. So maybe almost all matches I could see before occurred in same server as mine. Probably my case is different from yours.
BTW, actually I think friend support is a very minor part of T11 game. Moreover, I have about 100 friends in each of my T11 a/c and so I don't have much problem of lacking friend support. So I personally don't support requesting Nordeus to spend too much time on this feature. But I am glad to discuss your suggestions.