I think that could work, perhaps also have the travis/appveyor badges point to the instance used by the mirror and then it is less obvious.
Still it seems like a degree of obfuscation/indirection that is always a bit nervy. We can add the ability for the travis script to automatically run coverity scan as well. So may be worth the extra hassle. I am not sure how quick the mirror will update, but the nice thing of not doing that and being direct is that the PR's will tell us if the code at least passes the tests etc, before pushing. So again it is a bit more work to check the mirror instead of the PR itself.
I suspect there are manual work-arounds to admin priv for travis but not push access. Both of which I suppose are dangerous, unless we create another server to check the build or even move to reproducible builds (a lot of work). Debian / mozilla etc (Tor/bitcoin) have done work, but I am still not convinced this is foolproof either (
https://wiki.debian.org/ReproducibleBuilds )
I suppose the scary part is travis and other CI privileges, at least though as it is git in the background there can be more checks made that could catch any injection etc. These could be prior to packaging but perhaps not as part of CI.
Anyway sorry for the ramble, just some thoughts.