Well, I finally tried the solution posted earlier to get PI8.5 to work on the tabs.
I got the PIAgenda to open, but only when I pressed on an appointment within Manila- not on the lower left softkey where I had assigned the calendar.
And it also works to bring up PIContacts when I select All People on the Contacts tab.
I call that a success.
BTW- Using MightROM3.6 and loving it.
small update: With some tinkering with some of the "hacking" apps, I got my Home left softkey to work with New Event and it uses PI instead of the OEM calendar app. Very pleased.