Hi Nichandro,
I will put in that feature request for you.
Since sitelinks can be found on multiple levels, you will need to create multiple variables in the ENTITY_TYPE_INFO for each one (e.g., campaign sitelinks and ad group sitelinks).
Here would be an example on how to create one for ad group sitelinks.
AdGroupSitelink: {
selector: 'extensions().sitelinks',
asText: 'getLinkText',
level: 3,
parentType: 'AdGroup'
}