There have been freight only versions of all three 146 series. Not sure about QC - only ones I have seen were 200s. I do know of an Australian company that recently acquired two -200 QC s and would probably dry lease, trade in your whatever, or sell because wheeling and dealing is what they seem to do. Google it or PM me.
Wet leasing with Australian crews is not such a good idea, because our motto is " we may be rough, but at least we are expensive".
I am unaware of an STC to do a QC conversion of the RJ series, but why not ask BAE direct about that?