Roma's only grow to a certain height, so it wouldn't be worth trying to cordon them. Why not try for a cordon type that is similar? I'm sure I've seen some red plum toms that cordon in the catalogues.
I've grown Romas outside for two years running. They missed the blight last yaer, but have suffered this year. I think most people's have. My mum's have, even in her greenhouse. Might be worth giving them another go outside next year - if we have more favourable weather, they may not be affected by the blight. Or go for a blight resistant one.