The toxin in rhubarb leaves is mostly oxalic acid. There are some other compounds as well, but OA is the main culprit.
OA is also found in rhubarb stems (petioles) in lower concentrations than rhubarb leaves. Look up rhubarb on Wikipedia, the toxicity is explained there.
I note that before I started fencing my garden, deer routinely ate all my rhubarb leaves with no ill effects.
Bottom line, yes, you can safely compost them.