The attribute "async" is new in HTML5, therefore it was not part of the design, when the interface as developed.
I've added just now support for this attribute to the oacs-5-8 branch. Note that you have update acs-templating and the toplevel www directory (see [1]).
best regards
-gn
[1] http://cvs.openacs.org/changelog/OpenACS?cs=oacs-5-8%3Agustafn%3A20140512111718