Why don't you just wait say 2 weeks before the flight departs and then book your seat. Don't call Dubai. They can't help you. Normal res in the US should be able to do it. If its their system that doesn't allow seat selection on more than 7 weeks, then there is nothing you can do about it. TK for example can only assign seats on certain fares, and on certain sectors 3 weeks prior to departure.

Really.. what's the urgency to do it now?

OTOH, you can just call UA and ask them to request for a generic window seat if it pleases you. They know how to do so.