Upon further investigation, it looks like this standard library module is split out into the python-misc module.
One can infer how the features of interest are divided among packages by looking in the packages-split directory that is generated upon the completion of a Yocto build.
For my ConnectCard i.mx28 jumpstart board, the relevant directory to inspect is
Grepping the files there shows that <ast> is in the python-misc package. Therefore, one can modify the local.conf to include this package in the install. In so doing, the standard library support for the ast module is recovered.