What equipment is required to clean and fill a commercial keg (not one adapted for homebrewing), before paying for a full blown keg washer and filler? Is this even possible?
I haven't found removing the spear to be too much of a problem once you depressurize it (screwdriver in a rag pushed down on the ball) remove the circlip it just lifts right out.
Assuming you are only using it for bright beer (i.e. not fermenting in it) I would think you could get it reasonably clean by just boiling a small amount of water and PBW inside it although Jimmy is right that inspecting it will be a problem. Brewers hardware (link no longer available June 2021) has a great sanke conversion kit for making into a fermenter and discusses cleaning on the linked page.
Despite all this. I still think the cornelius soda keg is the way to go for home brewing. Six to the bbl, easy to clean, cheap to obtain and easy to move when full.
I use converted Sanke kegs for my HLT, MLT and BK and love them but will stick with Cornies for packaging and dispensing.
I'd look into making a homemade version of a spray ball or wand, that you can insert in the keg. Having it recirculate wash solution inside there.
Of course the more diligent you are about cleaning the keg after its empty the easier it is to clean. For the most part a seal keg just needs a good rinse with cleaner and sanitizer. Because there should be nothing but liquid remaining. Of course if you aren't filtering the beer then you are more likely to get some beer stone in there to clean out too.
In the interest of cost, most small scale start up breweries get good with fabrication in their own shop. This is a must if you can't afford to by pre-made and designed pro-grade equipment.
You could fill it through a regular sanke coupler. You'd have to remove the check valve and then connect connect whatever device you use to get the beer out of your fermenter. For example an auto-siphon with a tailpiece on the other end of the tubing.
To clean it, you probably want to remove the spear, which takes some tools. See here
Even then if would be difficult to visually inspect that all surfaces are clean. All in all kind of a pain.