I can definitely confirm that the Beolab 5 are controllable with the Logitech remotes (I have tried the touch ant the ultimate touch including the hub).
they need to be set in to the correct mode (therefore you need a beo4, changing the mode is not possible with the harmony), and everything works well.
Mike, I assume they're running in "option 1" mode? Based on that the Harmony should have commands like power on / off and volume? Can understand the need for a Beo4 to initially set mode as probably BL5 would be set mode 0 "powerlink data".
I currently have a Harmony Elite controlling a LG tv and s8 connection hub & BL9. Will probably be upgrading to BL5s and would be nice to have one remote - with a beo4 as a backup.