Hi there,
I apologize for not responding quicker. In all honesty, I am currently using Cubase 7 and have not updated my signature, sorry for the confusion!
Though I cannot be 100% certain, I am fairly certain you will not have an issue with HALion 5 and Cubase 6.5 and all of its functions. I was using HALion 4 with Cubase 6.5 and it worked flawlessly. HALion 5 is not much different at all in terms of how HALion works, just new stuff added. Perhaps someone can chime in specifically, but I don’t think drag and dropping of midi from HALion to Cubase 6.5 will be an issue as HALion 4 and Cubase 6.5 allowed drag and dropped to and from the two applications for samples etc. I have seen no mention at all about Cubase 7 being needed to full HALion functionality, in fact, HALion is not made to be a Cubase only sampler, it can work in other VST2/VST3 hosts and no limitations have been mentioned in using hosts other than Cubase.
Best of luck!