If I was a honeybee; I'd much prefer going down fighting rather than starving. Don't know for sure but suspect bees may feel the same. Starvation is no fun at all.
In my own experience, Open Feeding works very well "if" you can place it far enough away, at least 100 yards from the beeyard and if there is some sort of barrier (trees, fences, buildings) separating them, it works even better.
Feeding just "one" starving colony in the beeyard will usually encourage robbing by any others so If you must feed inside the hive "feed them all" regardless of suspected need. This alone Seems to keep all concerned occupied :) and unlike open feeding you can provide 'just enough' to keep the stronger colonies/robbers busy and 'more than enough' to those suspected of being light/starving.
When feeding a weak/light colony always REDUCE entrances to allow for 1 or 2 bees at a time.
Dry sugar, as MB suggested can work very well, but during warm weather bees may ignore it, including those close to starving. Getting it wet can help entice them.