My bad for answering questions before a morning coffee. The API spec states that it should be able to provide the version of tools, assuming a set is installed on the VM.
It works just fine in 5.5 for me, but tools is in a different section:
I did a get on /api/vApp/vm-67c4ecea-5d7f-4ed6-82d8-d415f84922bc
<RuntimeInfoSection xmlns:vcloud="http://www.vmware.com/vcloud/v1.5" vcloud:href="https://.../api/vApp/vm-67c4ecea-5d7f-4ed6-82d8-d415f84922bc/runtimeInfoSection" vcloud:type="application/vnd.vmware.vcloud.virtualHardwareSection+xml">
<ovf:Info>Specifies Runtime info</ovf:Info>
<VMWareTools version="9344"/>
</RuntimeInfoSection>
but I don't have a 5.1.x generation environment to look into right now.