I have run the engine with the rocker covers off for a short time, after fitting roller rockers to check all was working well. Keep an eye on the oil dripping off the rockers onto the hot exhaust!
If you have the later "plastic" type rocker gaskets they can be re-used several times without any sealing problems. The early gaskets are paper and tend to fall apart when disturbed. It's not easy to see what type you have until you remove the covers, although you can just see the edge in the joint above the exhaust, the later type are about 1mm thick and paper much thinner. Phil Bleazey did replacement rocker cover gaskets in paper I think. Not sure if the S&S later type gaskets are still available or not, they were not cheap I remember at about £35 each a few years ago. Not tried it in this position but silicone gasket sealant works well usually.
