Meringue

How about adding up/down conditions? Would of course cause a few more models.
Something like this (hope i get it right)...

iron_bars.json

{
    "multipart": [
        {   "when": { "north": false, "east": false, "south": false, "west": false, "up": false, "down": true },
            "apply": {  "model": "iron_bars_post_end_up" }
        },
        {   "when": { "north": false, "east": false, "south": false, "west": false, "up": true, "down": false },
            "apply": {  "model": "iron_bars_post_end_down" }
        },
        {   "when": { "north": false, "east": false, "south": false, "west": false },
            "apply": { "model": "iron_bars_post" }
        },
        {   "when": { "north": true, "east": false, "south": false, "west": false },
            "apply": { "model": "iron_bars_cap" }
        },
        {   "when": { "north": false, "east": true, "south": false, "west": false },
            "apply": { "model": "iron_bars_cap", "y": 90 }
        },
        {   "when": { "north": false, "east": false, "south": true, "west": false },
            "apply": { "model": "iron_bars_cap_alt" }
        },
        {   "when": { "north": false, "east": false, "south": false, "west": true },
            "apply": { "model": "iron_bars_cap_alt", "y": 90 }
        },
        {   "when": { "north": true },
            "apply": { "model": "iron_bars_side" }
        },
        {   "when": { "north": true, "up": false, "down": true  },
            "apply": { "model": "iron_bars_edge_up" }
        },
        {   "when": { "north": true, "down": false, "up": true  },
            "apply": { "model": "iron_bars_edge_down" }
        },
        {   "when": { "east": true },
            "apply": { "model": "iron_bars_side", "y": 90 }
        },
        {   "when": { "east": true, "up": false, "down": true  },
            "apply": { "model": "iron_bars_edge_up", "y": 90 }
        },
        {   "when": { "east": true, "down": false, "up": true  },
            "apply": { "model": "iron_bars_edge_down", "y": 90 }
        },
        {   "when": { "south": true },
            "apply": { "model": "iron_bars_side_alt" }
        },
        {   "when": { "south": true, "up": false, "down": true  },
            "apply": { "model": "iron_bars_edge_up_alt" }
        },
        {   "when": { "south": true, "down": false, "up": true  },
            "apply": { "model": "iron_bars_edge_down_alt" }
        },
        {   "when": { "west": true },
            "apply": { "model": "iron_bars_side_alt", "y": 90 }
        },
        {   "when": { "west": true, "up": false, "down": true  },
            "apply": { "model": "iron_bars_edge_up_alt", "y": 90 }
        },
        {   "when": { "west": true, "down": false, "up": true  },
            "apply": { "model": "iron_bars_edge_down_alt", "y": 90 }
        }
    ]
}

Confirmed for 14w31a.

Not only the transparency is important, it seems also important that the visible pixels have to be pure white.
My font had a very light yellow tint (artistic reason, not a mistake), this caused problems - seems that the boundaries weren't detected properly anymore (as you mentioned already).

Not a complaint! I just felt like adding this to your list. 😉

What does "non-0 alpha values" mean? I have a also a few stray characters that i can't get rid of no matter what i try.

Edit: Duh, got it - anything else than pure white and opaque will cause errors. Thanks, works perfectly! http://i.imgur.com/MY0lamv.png

Thanks a lot Grum! I assume by garbage you mean extra pixels in front of the letter images? Will a mcmeta file be needed for this fix?

Yes please, would be great if fixed soon. 🙂
This means that most texture pack authors will have to remove their custom fonts until then.

This happens also for non-HD (128x128px) custom fonts. It seems the spacing for the original "default.png" is used.