Inputs.io seem to have the ability to sign messages. Can someone confirm if it is ok?
You'd have to ask Tradefortress to be sure. But based on what I see on my own inputs.io wallet I'd say no.
We can confirm that the following wallets let you sign messages:
Blockchain.info,
Satoshi (Bitcoinqt)
Amoury
Multi-Bit
Electrum (Although seems to be a command line thing).