Andy, I realize you did a lot of changes in cometd-code in the last week (starting on 25.02).
Since last week users also report severe problems with SqueezePlay menus running iPeng or SqueezePad together with 7.5.4 or 7.6.x nightlies.

I did install 7.5.4 - r31995 now (on a Mac) and realize that the server indeed doesn't send any menuupdates to the clients any more.
This can be reproduced with SqueezePlay as well: if you try to turn on/off a SqueezeBox via the Controller then the menuitem-text ("turn XYZ on/off") does not update anymore. Guess other menuitems are affected as well.

SqueezePad and iPeng don't issue a direct menu request ("menu 0 100 direct:1") to retrieve the initial menu like SqueezePlay but instead expect the menu to be send via the "menustatus" subscription on the channel "/slim/menustatus".  Nothing arrives on this channel anymore (neither the initial menu nor any updates), so neither SqueezePad nor iPeng is able to render any menuitem.

It's easy of course to use a "menu 0 100 direct:1" request to retrieve the initial menu and I'm also willing to change my code if needed.
But with the missing menuupdates I'm a bit reluctant yet to submit any codechange to Apple as maybe this is a more fundamental problem in your current changes.

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After digging deeper in the code, I think the problem starts in CometD.pm:requestCallback (line 942).
In my logs I can see the line
"[11-03-06 15:31:16.6560] Slim::Web::Cometd::requestCallback (947) requestCallback got results for /3e03aa69/slim/menustatus/c8:bc:c8:97:98:5b / 4"

This refers to a menustatus-request (id:4) issued earlier:
[ {  channel => "/slim/subscribe",
    data => {
          request  => ["c8:bc:c8:97:98:5b", ["menustatus"]],    response => "/3e03aa69/slim/menustatus/c8:bc:c8:97:98:5b",
      },  id => 4,  }, ]

So the server still wants to send an event to the subscription, which is just not delivered. I can see that in this method you also did a change, trying to batch some responses. Guess this is where one should start to look if the batching actually works.