Unquestionably safer to dessicate. I mean to say the lead could be leached if one were of a mind to; I did indicate it's probably not a good idea.
No supercritical anything was sketched out however; just a reducing pyrolysis (making charcoal) and leaching it once or twice with nitric acid. Some lead would undoubtedly escape during pyrolysis unless the fumes were washed, which is tractable (just bubble it through the nitric acid and reuse it for the leachate) but this just adds to the already considerable expense.
Dessication is probably a safer way to reduce volume. I don't think lead is pernicious enough to warrant the cost of supercritical oxidation.