See Plan of Attack.

'Evasion ability'... uh-huh. Is trample an evasion ability? How about afflict? Menace surely is, right?

Referring to 'ability' would mean, as far as I know, that it wouldn't be limited to keywords. So would Ogre Marauder also be counted for this?

Well, the qestion with Ogre Marauder is easily answered by looking at the Oracle text. It's original printing is badly worded and cannot be unambiguously parsed anyway. :)

But it is true that evasion abilities are well-defined and many abilities that are only... uhm... virtual evasion are casually refered to as evasion abilities and this will hence be problematic with a lot of people not knowing the answer to the questions posed by Tahazzar or even wrongly assuming they know.

I didn't even realise it was an official rules term. The obvious fix is reminder text with those that exist in set. Which would sadly clutter the card.

Evasion ability does have a defined meaning in the comp rules. It's any static ability on an attacking creature that causes a restriction on what can block it. (Rule 509.1b)

For the purposes of these cards, I'd have to relax the "attacking creature" requirement, but I don't think that causes any problems

Whether an 'evasion ability' has a defined meaning in the comp rules or not doesn't matter if most of the players won't be sure what it actually means. Is it really worth delving into this? You could add some kind of reminder text here to help people out. With the same flavor idea, I would wager WotC would just name few or the recently used evasion keywords here (namely menace) instead of going with that poorly known 'evasion ability' term.

Oh sure, this probably wouldn't make sense to print in a real set. I was just exploring the space