Please let me know how it turns out!
Well, I tried playing around with this last night with partial success.
First, since I use the Mini+ largely for the IR remote capability, I attempted to get that functionality back. The Mini+ does not seem to work at all with the Peel remote that HTC has transitioned to as of midnight last night. If you try to use the HTC Sense TV app after the March 31 app update, it will not let you do anything other than download the Peel app. The HTC Sense TV app has been removed completely from Google Play. The workaround is to use an older version of the TV app. I had an older apk already saved, so I installed the slightly older TV app. This allowed me to still use the TV app on the M9 without being forced to use Peel.
Now, I wanted to get the Mini+ to work with the M9's remote capability. On the M9, in the /system/app folder, there is a folder called HtcAccessoryService. I replaced the HtcAccessoryService.apk inside that folder with the HtcAccessoryService.apk from my M8 running Lollipop, set the correct permissions, then rebooted the phone. Then, I paired the Mini+ with the M9 and the resources from the alternate HtcAccessoryService.apk were properly loaded to the Mini+. I had a "TV" menu item on the Mini+ and could select and load the various remote controls I had configured within the HTC Sense TV app. The only problem is that when choosing a command to perform on the Mini+ (like Volume + or Power On/Off), nothing happened. The IR signals weren't getting sent from the Mini+ at all, even though it was acting like it was working properly. I even paired the Mini+ with the M8 to verify the Mini+ wasn't broken, and it worked perfectly.
So, while things are kind of working, I believe there are still some libraries or something missing from the M9 that will need to be on there for things to work how I want them to. The TV and HtcAccessoryService APKs on my M9 are the same ones on my M8 and the M8 works perfectly. I'll keep digging and testing to see if I can get it to work properly.