Sitemap.xml lacks support for multiple (HST) sites under same host

We tried to use the sitemap.xml for onehippo.com, which has several HST sites under the same host, i.e. onehippo.com/en, onehippo.com/nl, ...

It seemed necessary to create separate sitemap.xml files for each site, because, otherwise, multiple (hst:)sitemaps had to be crawled in one go, and we didn't find a configuration option to do so.
Multiple sitemap.xml files can be combined by a sitemap index, for which there is some support in the plug-in. However, it appears that this support is limited to the situation where a single sitemap.xml file has to be split up across multiple assets (due to size constraints). If multiple sitemap.xml files are used, the plug-in's mechanism to combine them into a single sitemap index is too simple. This should be improved.