You didn't say which version of CF are you using, so Goyix's solution is partially correct: it works with the Railo, but not ACF.
In ACF8+ you can use the ServiceFactory to extract the real path. Code can look like this:
<cfset mapping = "/fusebox5" />
<cfset serviceFactory = createObject("java","coldfusion.server.ServiceFactory") />
<cfset mappings = serviceFactory.runtimeService.getMappings() />
<cfif StructKeyExists(mappings, mapping)>
<cfdirectory action="list" directory="#mappings[mapping]#" name="test">
<cfdump var="#test#">
<cfelse>
<p>Mapping not found</p>
</cfif>
Note: used my existing FB5 mapping for the testing.
EDIT
Proposed later method with ExpandPath is much clearer. Leaving this one only as possibly useful alternative solution.