Looking forward to this possibility when the wires are up after many years of travelling behind them on the WCML.
Are there any technical reasons why this could not happen ?
Probably not. Could turn up on a railtour. It would be dependant on signalling systems and the crews route/traction knowledge.
I don't think anybody has proposed electric freight on the GWML yet.
But nothing to stop an 86/87/90/91/92 running Paddington-Bristol (Parkway before Temple Meads) or Cardiff when wired.