Pitbull (which I believe Mr Mus has installed) can handle that part of it fine, I believe. It's an issue of a) having oozes targeted instead of Professor sometimes, so any cast bars related to your Target are not reliable, and b) like many Rogues, he appears to use Focus for Tricks of the Trade. Tricks target often changes from fight to fight, so a lot of people use a macro "/cast [@focus] Tricks of the Trade" and just change their focus for each fight depending on who needs it. Doing this, you can't use your focus' cast bar either.
Leaves having the tank call it out on Vent as the only remaining option that I'm aware of. Maybe DBM has an optional announce feature in raid warning when the cast starts. It's only a 2.5 second cast though, so when you add in Vent lag you don't really have a ton of time.
Though now that I think about it, you also have the time it takes the tear gas vial to fly through the air and hit the ground, too? Hrm. If that's the case, perhaps the addition of Vent lag to the process won't be so bad.
In only slightly related news, I really hate Queen. We've killed her 3 times and she's still not on farm. We murder Festergut in 4:21 and kill Queen as she enrages, on the 1 in a million attempts we don't mess up bites. I'm usually pretty proud of what my guild accomplishes in the number of attempts we actually spend, but 6+10 attempts over two raid days for first kill, 5 for second, then 10 for third is ...not something to be pleased about.
Farm status is elusive on her for some reason... at least for my guild. Alas.