"not compatible with linux"
Ain't no such animal!
Linux is an open source OS so can be adapted to almost anything that is remotely like a computer with enough effort. It has been ported to every RISC/CISC chip known. It has been ported to cell phones, to main frames, to digital watches, to video game systems, etc...
In this case I doubt you'd even have to go that far. It sounds like this is just a regular PC with a different kind of power source so your average PC Linux distro ought to install.
Power source shouldn't really matter from the OS' perspective unless you're using the OS to do specific power management (e.g. hibernate when on battery). If this uses standard CPUs, disk drives, memory and buses then I'm guessing you can successfully install Linux on it and ignore what the vendor told you.
Many organizations are only familiar with supporting (badly) things that use Windows so they just balk immediately when you talk about Linux. If you were to believe most Cable and DSL providers Linux and personal routers aren't "compatible" with their broadband modems. Despite that millions of us continue to use our own routers and Linux.
Don't confuse "not supported" with "incompatible". The one is telling you what the vendor will (pretend to) help you with and the other would tell you what absolutely does not work. If vendor support is important to you then "compatibility" isn't a key issue.