I got it open and fixed the lock late last night.
It case it ever happens to someone else, here's how I opened the trunk.
Deleted by Randy. Sorry, if anyone needs to know how to do this, pm me or a mod. I may be over reacting, but I want to be careful about telling the world instructions on breaking in. Thanks for understanding.
Next step was to repair the lock. It's fastened to the lid with 4 locknuts and the plastic cover is held in place with 4 plastic screws/anchors. The lock barrel had come apart (I think as a result of me pulling too hard on the emergency release handle earlier in the day) and little pieces went flying. I know better, and should have put a catch pan under my work; ended up having to retrieve two pieces from on top of the underbody sheet metal.
The lock barrel is attached to the latch mechanism by a short link rod (held in place by a cheap plastic clip). The shift rod had separated from the clip and that's why my latch wouldn't open. The latch mechanism is stamped with a Ford logo; little wonder that was the piece that failed! Obviously, no matter how hard I turned the key or pulled on the emergency release handle, there was no way for the latch to be actuated. Thus, pulling really hard on the release only caused a retaining clip to let loose, and this led to my lock cylinder spinning freely.
Somehow, I found all the parts (some of the springs and keyed parts inside the lock cylinder had gone flying), and put it back together. The retaining clip still works so I suspect it was never fully seated at the time of assembly. Just to be sure though, I used a drop of hot glue to hold in place. The plastic retaining clip that holds the lock barrel to the actuator and emergency release was a little scruffed but seems to be holding up OK, but I will probably ask for a repacement at my next service.
The engine cover vent is still stuck in place using the original adhesive tape, but will lift off easily enough in case this ever recurs.
Thanks to everone who offerred help.