Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
BAEC charges for each flight flown, so you are essentially redeeming for one ways; therefore, an open-jaw itinerary is not an issue. In fact, since UA permits one-way redemptions as well, you could redeem Avios for the outbound to Madrid, and UA miles for the return from either Lisbon or Porto on any of several Star Alliance carriers.
But Iberia Plus' redemption scheme is a little different than BAEC's, so make sure you investigate it before transferring any Avios to Iberia Plus.