Joe
If you built something that you can plug your hoses into with a small tank to hold the oil out of the attachment. You could have a ball valve at the outlet.Start with tank empty,bleed out the air and incomes the fluid. when you start getting fluid. your flushed.
If you found one of those old canaster filter housings with the drain on the bottom to drain old oil.Then your ready for the next time. Ive seen some of those fairly large. you wouldnt need the filter in the housing.
Mike
I try not to worry too much about this, I go out of my way to get the quick couplers clean. Rinse them off with brake cleaner and wipe dry.
As for the oil, its pretty tough to get it all out, you can get 90% of it out of a continuos flow attachment, but you will never get it all out of an attachment with cylinders. On a continuos flow attachment you could put a low pressure return line filter in the out flow to protect your loader if you want.
I don't rent my brush cutter anymore, but I also put a high pressure filter in the pressure line on the attachment to catch any dirt that may get in the sytem or any wear that comes from the skidsteer.
However most filters (high pressure and return) have a bypass valve, so unless you warm the oil buy running the attachment slowly, you may open the bypass and allow some oil to bypass the filter until it thins enough from warming to flow through the filter quickly.
I only way to avoid this is to have all your own attachments. Even then you can have the pump going on your loader and wipe out the motor on your attachment. Or the motor wearing out on your attachment and contaminating your loader.
I do believe in filters, the more the better, also change your hyd oil and keep a eye on it for water contamination. Keep you oil cooler clean and monitor the oil temp.
If your really fussy you can have you oil sampled by Caterpillar (and others I'm sure) and see if it contains anything it should not.
If you run equipment long enough, you will have a failure.
Ken