I've seen stuff done before where there is little clearance between the magnet and the wall behind it.

If the driver does not have a vented magnet, then it could probably be used OK. But if it has got a vented magnet then I'd suggest keeping more room because that air really can come out pretty quick from that hole, and if its restricted then it could cause the motor to heat more quickly. Also the air can 'bounce' off the wall behind it and back through the air vent and cause interferance with its operation if the clearance is very little and if the driver is used at high diaphram travel. Try putting your hand over the vent on your 2226's magnet, then turn it up high and you will see what I mean, you'll hear it actually.

Adrian