Cask-USA is an on-line group of cask ale enthusiasts hosted at Yahoo Groups, and moderated by cask maven Alex Hall.
A brewer, who had had a bad experience with a wooden cask, asked the group about the care of stainless steel casks. I offered this quick, not-too-terribly detailed, response:
Treat stainless steel firkins as you would stainless steel kegs... except, visually inspect, scrub out gross deposits, hose out (try creating a snap-on sprayball attached to a high pressure hose), and soak with hot caustic.
Talk with your chemical supplier. A built caustic (that is with added surfactants, detergents, etc.) is more effective, and ultimately more cost-effective, than straight caustic soda.
But be careful: there are some older aluminum casks floating about. Do NOT use caustic with aluminum casks. It will severely pit them.
Each time a cask is cleaned, or at least on a regular basis, use an acid cleaner to prevent the accumulation of beer stone.
And of course, after cleaning, fill the firkins with liquid sanitizer, seal, and hold for the time period suggested by the chemical supplier. Less effective, but gentler, would to rely on boiling water for cleaning and again for sanitizing.
As with not using caustic soda with aluminum, do not use chlorine based cleansers or sanitizers with stainless steel.