Nope. They have to be Flying or have Levitate to participate. If they aren't or don't have, then they cannot participate. From initial response not all Flying/Levitating Pokemon can participate either. They said there were some more restriction but never went into detail iirc. We'll know when the games come out though.
They did say that not all the Pokemon will be getting some.
Like I've said before we can pretty much rule out those that already have Form Changes since Mega Pokemon are also Form Changes.
Keldeo, Meloetta and others like Cherrim who mostly change while in battle seem like they'd conflict with a Mega Form. Those with temporary changes like Rotom, Shaymin, Deoxys conflict as you can change the forms outside of battle needing to create multiple Mega Forms for the pokemon which I don't see happening. The you have Battle Item forms for Genesect, Arceus, and Giratina which I again don't see happening.
Other than Pokemon who already have Form Changes, a few gimmick Pokemon I can't see getting Mega Forms, Ditto, Smeargle, Zoroark. Two of them defeat the point of the Mega Form Gimmick and one is an attack gimmick.
I see maybe 20 to 30 Pokemon getting Mega Forms and I feel as if I'm being too generous with this. I think its going to be a small collection and not a large amount.
