The MIT licence applied to all sample impls (of libraries that will end up in R7 Large) would be okay. (I actually thought this was already a requirement for SRFI sample impls.) Any similar licence, or strictly less restrictive licence, would also be fine, under the proviso of compatibility with the GPL, AGPL, and friends.
The MIT license is required for new SRFI sample implementations, but if there is an existing implementation that the author wants to include that is under another open source license, I'm willing to consider an exception. That is better than no sample implementation.