g:select with multiple preselected values

I'm using the g:select element with multiple selections enabled:

<g:select multiple="multiple" name='repositoryKeys' from="${Repository.list()}" optionKey='key' optionValue='name'>
</g:select>

This works fine. The problem I have, is with setting more than one preselected value. The value argument of the g:select element handles only a single string, no lists.

Is there a any workaround for this?

I couldn't find anything related to this on the issue roadmap. Multiple preselection looks like a major use case to me.

- Hans