Until the nation state tells a CA within their borders to issue a cert without publishing the CT logs, along with a gag order. If they don't comply they go to jail. You think Billy Bob Sysadmin wants to go to jail to protect a russian jabber server?
In the latter case, the malicious certificate + any CT log issued after that certificate should have been included are evidence of the attack that is easily verifiable and would likely cause browsers to drop the (edit) CT log unless provided with a very plausible excuse that isn't "we complied with a court/gag order".
Also, this would require the police etc. to compel 2-3 different entities: The CA and two log operators (I think one of them could be the CA itself).
Using different logs than normal would stand out, so it'd typically be two specific log operators that would have to be compelled to hide this. The US may get lucky and have jurisdiction over all of them, other countries are much less likely to.
a) Browsers aren't the only clients that matter. Will any XMPP client even look at the CT log? Probably not.
b) Browsers are not going to drop all US-based certificate authorities. And if they tried they might just get their own national security letter (reminder: all major browsers are US-based).
You can't work around the government using centralized infrastructure.
Browsers would happily accept it and IIRC they don't check against the logs.
As the other commenter already pointed out, Firefox does not require this. Safari and Chrome do. This indeed is not directly applicable to this situation, since XMPP don't involve browsers. But for websites, the parents attack scenario is not applicable.