I had a similar idea in the past but never created it because:
You cannot automatically send messages to chat.
You always need to pres the return key.
So for 1 item this will be ok but if you want to send a complete set info + text and itemlinks this will be more than 1 message and most likely annoy you too much as you need to press the return key in between each time.
Having other addons active which change chat channels automatically will also interfer and send the message to the wrong channel then, and whatever problems there could be or happen
If there would be a better way to communicate to groups like they have your same addon installed and can see the information somehow on their UI, send via MapPings (but there are plenty of addons already sending data with this "workaround" which I do not want to do as well as it might get you kicked due to too much messages send in too short time; and the other addons are like ultimate, dps and heal sharing which is more needed [at least in other ppl's eyes]).
I'd love if ZOs could finally create an addon chat channel where only addons are allowed to post and read data to/from, and maybe only out of cyrodiil and out of combat so no dmg sharing etc. is possible via this chat channel. This would be great as it would create SOOOOO many new addon possibilities like our idea here,
or sharing of role play texts, biographies, afk messages, "inspect other players complete gear and skills" etc.
Btw:
We managed to do those pre-trial questions and set choosing before trials via our forums, but if you run random this is impossible, yes.
Maybe use a notebook addon and copy&paste some predefined texts like "Stam DDs pelase write 'stam+' into group chat" and you can easily count and decide your sets then.