I used to be involved in with emulators in years past. I was learning how to develop games for GameBoy.
Sorry to burst your bubble, but I see 2 HUGE problems in attempting to run an emulator on roku.
First, as far as I know, all game roms are compiled code with lots of data (graphics & audio) embedded in the code. When I was attempting to build my own custom encryption scheme for roku, I found roku doesn't seem to be good for manually parsing binary data. It only seems to be good at parsing ascii data. I never was able to successfully decrypt a .png image file and display it on roku. So parsing a rom would most likely be impossible with the current SDK.
Second, even if it is possible to parse the rom file, the time it would take to parse the file would be longer than most people would accept. In my attempts at simple manual encryption/decryption, I found manually parsing small text files to be extremely slow.
If I were to attempt anything along the lines of what you are suggesting, I would look into building a "rom converter" that could be run on a computer. That would take an enormous effort to do. We don't even have a brightscript emulator yet. So I think one person building a program to convert a rom to a brightscript project is very far fetched.
YungBlood
Bringing more fun to Roku!