I have a VM setup that runs various Boinc projects, I installed Virtualbox in this VM but Rosetta keeps saying no hardware support.
As far as I can tell the hypervisor supports nested VM's (KVM on TrueNAS Scale), within the VM lscpu reports that Virtualisation is enabled but I cant get any units?
Does anyone with a similar setup manage to get it to work?

I ran into this problem as well (on Hyper-V). The issue is that BOINC checks /proc/cpuinfo at startup and seemingly doesn't enable VirtualBox when it detects the 'hypervisor' flag in lscpu. Since you're using KVM as the hypervisor, you can set the "hidden" attribute if you're using libvirt XML, probably something like

<kvm>
      <hidden state="on"/>
</kvm>

or otherwise set that KVM option, and it should work.

For me, since Hyper-V doesn't seem to have a way to change that, I patched the BOINC executable to read from a different path instead of /proc/cpuinfo where a copy of /proc/cpuinfo is that has the 'hypervisor' flag removed. In hindsight I probably could have just bind-mounted /proc/cpuinfo to that other file, maybe in the systemd unit for BOINC, but whatever.