From what I've read, it supports $name$ and the name of the saved search, and $seach$ as the search string itself.
You can configure scripted alerts as per as alert_actions.conf#Script_options, but you're constrained by these 8:
SPLUNK_ARG_0 Script name
SPLUNK_ARG_1 Number of events returned
SPLUNK_ARG_2 Search terms
SPLUNK_ARG_3 Fully qualified query string
SPLUNK_ARG_4 Name of saved search
SPLUNK_ARG_5 Trigger reason (for example, "The number of events was greater than 1")
SPLUNK_ARG_6 Browser URL to view the saved search
SPLUNK_ARG_8 File in which the results for this search are stored (contains raw results)
If you want to use hosts, then I think you're going to have to play around with custom scripted inputs.
Check this link out as well, which can point you in the right direction: http://splunk-base.splunk.com/answers/34570/how-to-add-custom-email-alert-content
