On pg. 6 of the HQueue Cloud guide:
By default, it is not possible to create a custom Houdini setup using your own HDK plugins and a specific Houdini version. If you are an Annual Upgrade Plan customers and you are interested in running your own instance using a custom Houdini setup, then please contact your account manager or email us at firstname.lastname@example.org to discuss what is possible .
So it seems that it's possible.. just a bit more work.