The main point in favor of having IncludeOS on top of Solo5 is that Solo5 would hide a lot of details. For example, the hypercall mechanism is already different between architectures: ukvm in x86 uses PIO and arm uses MMIO. The ukvm interface might also change, specially ports and the mechanism used to implement hypercalls. Another point in favor is that by coding against Solo5, IncludeOS would get muen; and ukvm for arm, freebsd vmm, and macos for free.
The issue with having IncludeOS on top of Solo5 is that both Solo5 and IncludeOS implement many of the same features: memory management, interrupt handling, startup, virtio (if Solo5 is built in virtio mode). The cleanest approach would be to use the Solo5 version of all of these, but IncludeOS have some of those enhanced for better performance (like virtio). So, we would have to use a complicated mix from both sides, or disable IncludeOS on solo5_virtio.