Is there a reason why ESXi 5.5 does not support RDM? A lot of people ask for it and use it so I have no idea why VMWare doesnt work on this feature to implement it and support it natively, at least from a software point of view
Just because "several N54L owners" have got it to work, does not mean that the controller is supported. The Controller is not on the HCL this means one of two things:
What you are doing is not a normal use case. With local disks there is usually no requirement for a RDM,
in fact there few use cases for RDMs left:
I am assuming that as you are using a HP Micro Server this is not a production environment but your home lab.
You have answered your own question, it seems that according to the KB your controller must lack the required functionality to support this feature.