For all their cons, they do seem to be nice rugged boxes that work fine as only and you're not going to do constant heavy AES.
Regarding HDMI: you could just disable those ports and maybe desolder them. Plug the holes, add SGABIOS via an OpROM loader and do everything using RS-232.
The problem with those rugged integrated kits is that there are as far as I know always purpose-built which makes repurposing them for something else either slightly flawed or in need of some hardware hacks.
Now, since you are talking about MiFi, I can only assume you are going to be mobile with this thing. The fact that it has a DC power jack makes that pretty easy, as you can basically use any common power source, including batteries. Due to its size and lack of break-off parts (it's pretty much just a brick), it'll probably live quite long as long as you don't take it for a swim.