Quote from: dmcdad on August 08, 2017, 07:46:20 PM

TLDR: Is it possible for Electrum to sign unsigned transactions created by Electrum Cash?

TLDR: No.

Long Version:
The BCC devs changed the SIGHASH algorithm for BCC to provide 2-way replay protection... so a BCC transaction is not valid on BTC network... and a BTC transaction is not valid on BCC network.

This has already proven "problematic" for peeps with Electrum 2FA wallets, who created ElectronCash 2FA wallets (using the Electrum 2FA seed) and attempted to spend coins. TrustedCoin doesn't support BCC, so they were signing the transaction as a BTC transaction and the BCC network would then reject the transaction (wrong SIGHASH).

This is effectively the same situation that you're describing... attempting to sign a BCC transaction (from Electron Cash) with a BTC sig (from Electrum)

Also, just to help avoid confusion, the BCC wallet that was forked from Electrum is now called "Electron Cash"