Nothing in the tablets realm exists for that kind of functionality as it stands right now. Almost all the tablets are HDMI out.
However....
You might be able to pick up an external DSLR viewfinder with an HDMI input that would work to fit your needs. It's basically a camera accessory for really powerful digital single lens reflex cameras. It isn't what it is intended for, but it might work. Range is $100-200, so it might be a better bet just to go for a PS Vita.
Hope this helps!