I've been messing with home-assistant.io and it's emulated hue bridge along with some other home brew skills. It has harmony hub integration so you can already setup basic device commands but that would end up being pretty trying. 'Alexa turn on scroll down' and the list moves down one step.
What would be needed is a skill that can access probably the mp-extended back end to read the databases and understand 'Alexa ask MediaPortal to play ...' and then it appears on screen. Unfortunately I think it's quite bit of work.
I have written a couple Alexa skills. I suspect having the proper WiFiRemote commands sent to MP would be very doable. You would just need to setup a port forward from the Amazon server to your MP computer in your router/firewall. I'm going to have a look.
Oh, that wifiremote seems to use that Apple Bonjour service, ugh. Hopefully MP can respond to simple HTTP commands like the Roku does