I’m guessing that where you say “model by-laws” you mean your own – the model by-laws have no standing unless they have been adopted by your strata plan.
The written permission issue is tricky – can you actually officially permit owners to park in visitor parking? Isn’t it then no longer visitor parking?
Permission for residents to park in visitors parking would be contrary to your own strata plan and by-laws and possibly the terms under which the DA for your complex was agreed. The written permission issue is relevant to, say, residents who want guests to stay longer than the normal permitted period for parking in visitors’ parking.
Most buildings allow a little leeway for residents to stop in visitors parking for a few minutes while they drop off shopping or whatever, but I’m pretty sure you can’t formally allow residents to park in visitors parking just because it suits them to do so. Any agreement by the EC to allow this would therefore be incompetent.