If its protection from o2 exposure thats driving this use suitable pressure retaining tube/hose and attatch to a corny black (product out) disconnect to feed into a corny and use the prv or a gas in disconnect with an optional bleed valve attached to allow presure out and the flow to commence.
If o2 exposure isnt an issue its easier to drain in thru the lid imho.
In Both cases however if you wish to retain condition during transfer chill the source well below pressure retaining levels, right down to 0c if you can, as this will counter any agitation in the transfer that would normally 'shake' c02 out of suspension.
Even with chilling right down you can expect some fobbing especially if closed transfer into a corny via the post n poppet bottleneck, so stand the recipient keg in a trub, and have a garden sprayer at hand to help knock back any foam that threatens to climb out of your trub.
To fill a minikeg a length of 10mm tube (or 3/8" od beerline) with a ptfe wrap along with a small rigid tube akin to a wd40 feed tube can be shoved to seal into a grey bung so you can prime with co2 and fill thru the 10mm tube into the minikeg base bleeding off gas and foam thru the lil rigid tube.
also try to prechill the kegs/minikegs in advance too..
