In think once you get the application to a certain level of completeness and popular usage, you will be in a good position to approach Instagram and ask about the official permission for this feature.

Given that they allowed/partnerred with Hipstamatic to upload directly from that app on iPhone, it is theoretically possible they could allow other applications to do so in future. However it is also likely that they will be very fussy about which applications they allow the direct upload feature.

Also, thanks for making this app!