This is probably obvious, but anything listed in the vCenter/ESX(i) compatibility list for supported guest OSes can be made into a VM. Of course, VMware Studio may not be the tool to use to create those VMs. If you are going to deploy RHEL4-based machines for example you aren't going to use VMware Studio. Sounds like you are in that sphere of operations to me. With RHEL4 a kickstart-based VM had to be created (no VMware Studio). YMMV...
So basically, the only way to package a Virtual app for a Linux OS that is not supported by OVT Tool or VMware Studio is by constructing the VM to self configure on first bootup and then export the VM into OVx format using ESX host?
I just want to know if there are any other tools out there that will support Monolithic Kernels like FreeBSD?
Thanks again for all feedback.