Can you get any skin in Mystery Skins?

You can get nearly any skin that's not limited or locked away for unknown reasons.

The remaining limited skins are:

• The Victorious line (Gold+ at the end of each season): Jarvan, Janna, Elise, Morgana
• Championship Riven (the last limited skin)
• Riot Singed: I forgot about this one. For some reason this is the only one from the Riot line that's limited. The rest are legacy meaning they can be back.

Then there's ones you can't get from mystery gifts, but aren't technically limited:

• The PAX line: Rumour has it they had a contract with PAX to not sell these, I don't see why that would stop them from giving them away for online events though. Lame.
• Skins given away for beta testing, pre-ordering, or simply being around in Season 1. Some examples are: Black Alistair, Human Ryze, Silver Kayle, King Rammus, UFO Corki, Judgement Kayle, and I forget the rest.
• I don't think you can get Rusty Blitzcrank either. It was removed for being horrendous and for that reason they've kept it removed.

Ahh, my bad. Either way, though, it's still not available through Mystery Gift.