Error: The Fitter cannot add location assignment PIN T28 to node

First determine if the problem is from the LLR or the virtual pins assignment. My guess is it's the latter. If so, go to the Assignment Editor, find all the pins with the Node Finder, and make virtual pin assignments here. This is what I've always done and had success with. (Usually you don't want a bulk virtual pin assignment, i.e. clocks are real, perhaps a global reset, etc. That's not to say your case may not have those conditions, just why I've done the more manual way in the past). If that works(or whatever works), please file an SR, as what you're doing should work too.