There is no online version of the ACU available for ESXi, but you can use the offline version - see my blog post here.
This requires a host downtime of course, so I really also like to see an online version for ESXi.
However, this is something that we need to request from HP, not VMware. They already have two command line utilities in their Offline Bundle (a Bootconfig tool and the Onconfig tool for configuring iLO), they just should add a ACU CLI ...
- Andreas