You just missed them. Not to diminish your inventiveness, but pipe foam or similar horn 'lips' experimentation has been posted about on various forums for over a decade with quite a bit done after Charlie Hughes (Peavey, ex-Altec?) 'invented' the foam terminated 'quadratic throat waveguide' around 2000, then more recently when 'Patrick Bateman' and/or one of his several aliases re-invented it for some very interesting DIY horn designs for his Honda using soft rubber terminations IIRC. I assume a Google search will find them all.

Really, all these are functionally just variants of Henry Kloss's felt ring around the AR speaker's tweeter and to a lesser extent, the offsetting of drivers on a baffle to average out its eigenmodes, though I don't know for sure who 'invented' this variation.

Regardless, glad to see someone besides me around here taking a 'hands on' interest in selective damping of a horn's internal reflections (AKA Dr. Geddes's higher order modes (HOMs)).

Absolutely, so many come to the DIY forums with their 'hands out' to primarily save $$$, but for me it's always been about getting the performance I want instead of just accepting the status quo of a particular price range.

I haven't done, nor am I aware of, any cloth covered foam/whatever termination experimentation, but it seems reasonable to me that it would mimic felt to some extent with the right cloth weave, so probably worth trying.

WRT damping the vanes, these act as HF dispersion reflectors, so any damping of their flanks will reduce off axis HF response. Since you're using a super tweeter though, wrapping them with absorption along with damping the vertical internal sides of the horn to at least the same depth as them same as me and a few others have experimented with might improve its clarity through the XO BW if you can find the right absorption factor to cover it.

The global approach of tightly stretching double knit cloth over the horn mouth has subjectively sounded good to me, so hope to pursue it further along with the two stage foam plugs currently in the 511s once I get them back on line.

Right on! A lot of horn 'honk' is due to all these various reflections, though IME, throat restriction is the worst villain and why one either has to XO a typical horn well above its design cut-off like was done on the M19, 9842 and later A7s along with the selective damping or swap it for a pure WG and foam plug, both of which still need CD horn EQ if used wide range with no super tweeter.

GM