Unnamed arguments are passed into a function as a structure with numeric keys matching the positions of the arguments defined in the function arguments. So instead of passing the named arguments, you can convert your array to a struct with numeric keys, and then pass the struct in with argumentCollection:
<cfset arr_arguments = {"1"="a","2"="b","3"="c"}>
<cfset someFunction(argumentCollection=arr_arguments)>
You can easily convert an array to a struct with numeric keys like this:
<cfset args = {}>
<cfloop from="1" to="#arrayLen(arr_arguments)#" index="i">
<cfset args[i] = arr_arguments[i]>
</cfloop>