Since I started this thread, I will report my experiences:
When I considered the hassle and potential problems in modifying an SPD cleat, I just never really tried.
I guess the bottom line is that I gradually got used to having no float and stopped worrying about it. As mentioned before, my knees don't really need float, it is just something I felt like I wanted.
I am now using two pairs of shoes interchangeably: one with yellow cleats and one with the blue cleats. I actually prefer the blue cleats. The float is so minimal that it feels like none.
In fact, the yellow cleats feel like no-float also still. But although it is not "float" (as in free motion of the foot) you can move your foot to a different angle if you try. So, ironically, because I can move my foot I do try different angles an I am never quite confident it is in the "right" place. With the blue cleats, I can't move so I don't even try and forget about it.
The smooth, stable, quiet, no-hassle operation of DA pedals has really won me over. Some things are worth paying a little weight penalty for. When I have to replace my cleats I will go with either Blue or Red.