iBike is an intriguing option. Since tandems don't draft as much as solo bikes, it would likely work relatively well, although it won't separate total power.
There's spider-based power meters and crankarm-based power meters. Crankarm meters should work for both stoker and captain, but the only 2-sided option here I know is Pioneer. I'm not sure how well it works, but avoids the assumption of L-R balance of Stages (which I wouldn't use). If it does work it would be perfect. One pro team (Blanco, formerly Rabobank) is using it.
There's still Garmin Vector on the horizon. They've been plugging away at the project and have announced something will be available this summer. Time will tell, however. Garmin's limited to Exustar pedals, though.