Few notes for your thinking.
First you can already test web pages threw squad MOTD pages. simply make the first line url in the text file you upload.
2nd there are many hooks into AH that are call like javascript , things like send dot command.
3rd don't forget there is also a swing out page available. Also selectable from the web content header.
HiTech
I'm not familiar with the squad MOTD. I've never seen what that looks like. If it is a modal dialog that pops up and covers the UI and blocks the user, I think that would be sub-optimal. I think like Spikes said, people are trained from web popups to just immediately close that out without looking at it and reflexively being annoyed. It would basically be immediately blocking them from what they came to do which is a rude thing to do to a user.
A swing out page is a reasonable alternative. It definitely puts the information directly in their face on startup, but doesn't block them from doing anything else with the UI and doesn't break them out of the context of the game client. However, I expect there would ALWAYS be some up-coming event, so you would have to be ok with that always opening up. That would probably be a solution that has the least impact on your existing UI layout. Sort of side-loading the content.
The final hat-trick would be to save the user the hassle of having to create a separate account on a separate website and be blocked from registering until they alt-tab out of the client to go to their email to click on the validation email link. Uhhhg. Could a hook be created like "GetGameId" that can be called by the webpage and causes a login validation on the client and returns the validated player id to the webpage? That would a
HUGE improvement to the user experience. Hmm there would probably need to be some kind of encryption of the return value so the webpage can't spam externally and knows that value is coming from HTC. Or maybe that call takes some kind of encrypted token so you know it is a trusted webpage. Probably the latter so you can be in control of your own security.
Thanks for keeping an open mind enough to game it out a bit.
[edit] One last though. The swingout page is probably the superior solution. That way if for some reason, the SE doesn't have information to share or the team is dismantled, or the SE server goes down, then the web content would be empty and the panel would just not swing out without altering the rest of your UI. $0.02.