I don't believe I've seen this as a suggestion ... I know with the quick reply box in the upgrade, this might not be *as needed* ... but one of my members asked me about it ... so I thought I'd ask you ...

Currently, in PMs, there is no ability to "reply with quote."

How difficult would it be to change this?

It's untested, but try ripping the code out of Quick Reply for 6.6 for the reply with quote; it's seperated from the rest since some people did/didn't want it...