[colinicky]

Unless you have a membrane compressor you would need a J of O2 anyway
In the op it stated :-
" For discussion let's say that the bank is 3 x 47l run at 250bar and cylinder WP is 232bar "

47L means J size anyway so get some "spare" O2 J's & these are already tested & cleaned so were is the problem ?
I am in the process of setting up a bank for our club & that is only for air ,so I just dump the appropriate amount of O2 in & air top .
The only premix I do is a J of 18/45 which I use all the time for my ccr but I can scavange all my J's as I have a booster pump so typically I send J's back with 5bar or less O2 in but unless you are doing a massive amount of fills or you need 200 bar of O2 fills it isn't worth a booster pump IMHO