http://www.webervst.com/blocker.html Does someone have experience with these? - seems like a good idea.
I think it might be a good way to get a better mids dispersion for bass (i play a tweeterless cab). Lows dispersion is never a problem anyway.
They'll work OK at frequencies where they are a wavelength or more across. For the four inch that's 3.4kHz and higher, for the five inch 2.7kHz and up, so OK for guitar, not so much for bass as Mark said.
You indeed are correct about the dispersion frequencies, but maybe if these things were used in front of a well-designed wizzer coned 10" midwoofer... Although, with a dome shape I think wave reflections back at and into the speaker would be pretty articulation-smearing. If it were more cone shaped and longer....and had waveguides on the outside edges of the speaker....that starts to be a good idea....wait, that's becoming a horn!