Whonix is known to work on the following hypervisors: Qubes (Xen), KVM, and VirtualBox. I don’t know how well any of them are supported on Trisquel.
The T400 can apparently run the current version of Qubes 3.2. (HCL). However, lack of EPT/SLAT support on the Core 2 Duo will not allow it to run the soon-to-be-released Qubes 4.0. Version 3.2 will be supported for one year after the release of 4.0. Also, keep in mind that Qubes is based on a Fedora dom0 so that may disqualify it for you.