I figured it was possible to do a grapple with an evocation block, and the Orbius spell on YS294 confirmed it.
A regular block involves the attacker actively spending his actions each turn to keep his target grappled. But it doesn't say so on the Orbius spell.
Would you rule that a wizard needs to actively concentrate on his target to maintain the grapple, or do you feel the extra shifts spend for duration on the spell covers it and allows the wizard to do some other actions on his following turns? On one end, it seems overpowered that a wizard can completely incapacitate a target for several rounds while acting. On the other end, the amount of shifts put into this long term grapple, especially given the chance he victim can break free anytime, could cost you a great deal of mental stress.
Also, could you duplicate a similarly incapacitating, but maybe not damaging, version of this with a maneuver evocation instead with an Invoke for Effect? After all, someone also needs to roll each turn to break free of the effects of a maneuver's aspect.