hiya
It is harder to carbonate a 50l keg in a rush than clean it to be honest. One way to clean without alot of equipment is to push the cleaning/sanitizers around with pressure from another keg like a cornie. Whilst exposure of caustic to CO2 will cause beer stone and neutralize the caustic, a quick push for a few mins will not give much grief.
Some of the big buts if you are using caustic is a) safety eye wear b) gloves and shoes (sounds strange but how many HB'ers brew in safety things) c) vent the keg of CO2 before starting. Caustic has a magic ability to find your eyes ... A rinse cycle is best between the caustic and Acid sanitize.
Do not use iodophor of any of the phosphoric sanitisers as any residue left in the keg stem or keg will taint the beer. Hence the reason the big kids use Peracetic Acid (PAA). PAA also will neutralize caustic with no flavor taint.
The special cleaning coupler comment has thrown us a bit, no such thing. One way valves are used when banking kegs in cellars, but in our 3 cellars we never bank so i rip them out. It is also way easier to carbonate down the stem than thru the gas port so the droppers need to be removed anyways. If you look down the piston you will see a plastic dropper and holding cage if present.
The mention to Sanke is also slightly misleading. The Americans refuse to use the proper coding system for kegs, which is A Type (Flat/Tooheys style), D (Well/CUB/Sanke), S (Asahi/Deep Well), G (Guinness/Trilobe). All work the same really, but A type is the most hygienic and easy to use.
All couplers have a 5/8'th thread on them and suppliers like Keg King and Andale sell conversion parts to a Cornie keg gas and beer snaplock fittings typically used in home brewing. Be careful on transfers of hoppy beers though, these fittings easily clog with hop flakes.
Happy cleaning.